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2025 OCR A Level Computer Science H446/
Computer systems Combined QP & MS
Accumulator (ACC) - answer-Holds the results of calculations performed by the ALU. Checks for conditional branching. Stores data from the MDR/RAM. Address Bus - answer-A unidirectional pathway that carries memory addresses for data or instructions. Arithmetic and Logic Unit (ALU) - answer-Handles arithmetic calculations and logical operations required for executing instructions. Buses - answer-Communication pathways that transfer data, addresses to other components and controls signals between CPU components. Contemporary Processor Architecture - answer-A modern CPU design that integrates elements from both Von Neumann and Harvard architectures. Control Bus - answer-A bi-directional pathway that transmits control signals sent from the control unit to manage the execution and coordination of instructions in the CPU. Control Unit (CU) - answer-Sends control signals to coordinate the movement of data through the processor. It also controls the buses to manage communication between the CPU, memory, and input/output devices. Current Instruction Register (CIR) - answer-A register that holds the current instruction being processed, separating it into operand and opcode. Data Bus - answer-A bi-directional pathway that carries data between the processor and memory. Fetch-Decode-Execute Cycle - answer-The sequence in which the CPU retrieves, interprets, and executes instructions. Harvard Architecture - answer-A computer architecture that separates instruction and data storage into distinct memory units to improve efficiency. Uses data and address buses. Memory Address Register (MAR) - answer-Stores the memory address of the current instruction or data location to be accessed via RAM. Memory Data Register (MDR) - answer-Holds data that is being read from or written to memory (RAM) during processing. Sends data/instruction to the current instruction register.
Pipelining - answer-A technique that enables overlapping instruction execution by fetching the next instruction while processing the current one. Program Counter (PC) - answer-A register that stores the address of the next instruction to be executed. Registers - answer-Small, fast memory units within the CPU that temporarily store data, instructions, or addresses. Von Neumann Architecture - answer-A computing model where data and instructions share the same memory and communication pathways. Not as efficient as Harvard. Complex Instruction Set Computer (CISC) - answer-A processor architecture designed to execute complex instructions. Instructions may take multiple clock cycles, instructions are variable in size in format, limiting pipelining efficiency. Complex circuitry increases cost in comparison to RISC. Graphic Processing Unit (GPU) - answer-A specialised processor containing many small cores optimised for parallel processing, commonly used in graphics rendering, machine learning, and data processing. Multicore Systems - answer-A processor design that integrates multiple CPU cores onto a single chip, enabling efficient workload distribution and improved performance. Parallel Processing System - answer-Multiple processors or processor cores work simultaneously to execute different parts of a task or multiple tasks at the same time, improving efficiency and reducing execution time. Reduced Instruction Set Computer (RISC) - answer-A processor architecture that executes simple instructions within a single clock cycle (higher battery life compare to CISC), allowing for pipelining but requiring more RAM for program execution. Less complex circuitry reduces cost. Fetch - answer-The PC copies the address to the MAR, which is then sent via the Address Bus. The PC increments, a Read signal is sent via the Control Bus. The RAM places the instruction on the Data Bus, transferring it to the MDR, then into the CIR for decoding. Decode - answer-Interprets the fetched instruction. The CIR sends the instruction to the Control Unit, where the instruction is split into opcode and operand. The CU then generates control signals to prepare the necessary registers, buses, and ALU for execution. Execute - answer-Carries out the decoded instruction. The result is then stored in a register, memory, or sent to an output device.
Operating System - answer-A collection of software that manages hardware resources, provides a user interface, gives a platform for applications to run on and controls access to a computer. Basic Input Output System (BIOS) - answer-Firmware stored in ROM that initializes hardware during the boot process and loads the operating system. It performs POST (Power-On Self-Test), checks connected devices, and provides low-level control over hardware components. Device Drivers - answer-Software that allows the operating system to communicate with hardware devices like printers, keyboards, and network adapters (peripherals). Memory Management - answer-Allocating and managing RAM (main memory) and virtual memory efficiently to allow multiple applications to run at the same time. Function of an operating system that allows for multitasking and security (does not let programs access memory reserved for other programs). Paging - answer-Partitioning memory into equal fixed sized divisions/pieces. Uses physical addressing. Can lead to waste space in memory if applications do not use the entire division/page. Segmentation - answer-Partitioning memory into variable-sized logical sections to optimise performance. Uses logical addressing, more efficient use of RAM compared to paging. Virtual Memory - answer-A technique that extends RAM using secondary storage, allowing larger applications to run by swapping inactive data between RAM and disk/secondary storage to make more room for active applications/data. Frequent swapping can cause disk thrashing. Virtual Machines - answer-Software that emulates a physical computer, allowing multiple operating systems to run independently on the same hardware. Useful for testing software without having the physical hardware required. Saves space, time and money. Scheduling - answer-Allocates CPU time to multiple running tasks, ensuring efficient execution. First Come First Served, Round-Robin, Shortest Job First, Shortest Remaining Time, MLFQ. First Come First Served (FCFS) - answer-Processes are executed in the order they arrive, like a queue. Non preemptive (once a process starts executing on the CPU, it runs to completion). Round-Robin - answer-Each task gets a fixed time slice before moving to the next, ensuring fair CPU usage. Preemptive (the CPU can interrupt a running process). Shortest Job First (SJF) - answer-Prioritises tasks with the shortest estimated runtime. Non preemptive (once a process starts executing on the CPU, it runs to completion). Shortest Remaining Time (SRT) - answer-Prioritises tasks with the least time left to execute. Preemptive (the CPU can interrupt a running process).
Multi-level Feedback Queues - answer-A scheduling system that organises tasks into different priority levels, allowing processes to move between queues based on execution time. Preemptive - answer-The CPU can interrupt a running process. Multi-tasking Operating Systems - answer-Supports multiple applications running simultaneously by managing CPU resources efficiently. Multi-User Operating Systems - answer-Supports multiple users accessing system resources simultaneously, managing user sessions. Distributed Operating Systems - answer-Multiple computers work together as one, distributing tasks efficiently across different machines via a network. Real-Time Operating Systems - answer-An OS designed for real-time applications that require immediate responses, such as medical devices and industrial systems, ensuring guaranteed response time with plenty of overhead. Embedded Operating Systems - answer-A specialised OS designed to run on dedicated hardware with minimal resources, such as appliances (washing machine) and embedded devices, optimised for specific tasks and low power consumption. Interrupts - answer-A mechanism where hardware or software sends an interrupt request to the CPU, pausing the current task to handle urgent or high-priority events. Interrupt Service Routines (ISR) - answer-A routine where the CPU saves its current state (storing current register values), executes code to handle an interrupt, and then restores the state to resume the previous task. Intermediate Code - answer-A partially translated, machine-independent form of a program generated by a compiler, enabling portability across different architectures before final machine code generation. Applications - answer-Software programs designed for end-users to perform specific tasks. Examples include word processors, web browsers, spreadsheets, desktop publishing and graphics manipulation software. Utilities - answer-System software with one purpose that is used to perform maintenance/upkeep tasks, such as improving performance, diagnosing issues, and managing files, to support the operating system. Examples include disk defragmentation, compression, backups and encryption. Translator - answer-A general term for software that converts source code from one programming language to another, such as compilers, interpreters, or assemblers.
Extreme Programming - answer-An agile method emphasising short cycles (sprints), frequent releases, paired programming, and continuous user feedback integration. Focuses on creating high quality code. Rapid Application Development - answer-A method for fast software delivery, using prototyping, extensive user involvement, and flexibility to adapt to changes. Spiral Model - answer-A risk-driven hybrid model combining iterative and structured development, refining prototypes through cycles with risk analysis. Waterfall Lifecycle - answer-A linear, sequential development model where each phase (e.g. analysis, design) completes before the next, offering structure but little flexibility. Programming Paradigms - answer-Different approaches to programming and computation (e.g. procedural, object-oriented), selected based on problem requirements. Procedural Languages - answer-A high-level paradigm where code is organised into reusable procedures and functions, using defined data types and control structures. Object-Oriented Languages - answer-A paradigm where programs use objects, instances of classes, encapsulating attributes and behaviors for modularity and reuse. Classes - answer-A blueprint defining attributes and methods that objects of a specific type inherit and share. Objects - answer-An instance of a class, holding specific attributes and behaviors as defined by its class. Methods - answer-Functions or procedures within a class, defining the actions an object can perform. Attributes - answer-Data variables within an object, representing its properties or state. Encapsulation - answer-An OOP principle that hides an object's attributes, restricting direct access and enforcing interaction via methods. Setting an attribute to private - answer-Achieves this. Inheritance - answer-An OOP mechanism where a subclass inherits attributes and methods from a superclass, promoting code reusability, saving time. Polymorphism - answer-An OOP feature allowing different classes to implement a shared method uniquely, enhancing flexibility and reuse. Allowing a class to inherit a method, but making that method do something slightly different.
Assembly Language - answer-A low-level language using mnemonics to represent machine code instructions, tied to specific hardware architecture. Direct Addressing - answer-An addressing mode where the operand specifies the exact memory address of the data to be accessed. Immediate Addressing - answer-An addressing mode where the operand is the literal value to be used directly, not a memory address. Indexed Addressing - answer-An addressing mode where an index register modifies a base address to dynamically locate the operand in memory. Indirect Addressing - answer-An addressing mode where the operand points to a memory location holding the address of the actual data. Lossless Compression - answer-A compression technique that reduces file size without losing any original data, ensuring perfect reconstruction of the file. Run length encoding and dictionary coding are examples. Lossy Compression - answer-A compression method that removes non-essential data, reducing file size but causing some loss of accuracy. Dictionary Coding - answer-A lossless compression method that replaces repeating patterns of text with unique codes stored in a dictionary. Run-Length Encoding - answer-A lossless compression technique that replaces sequences of repeating data (e.g. identical pixels in an image) with a single value and its repetition count. Symmetric Encryption - answer-A technique where the same key is used for both encryption and decryption, requiring secure key exchange. Asymmetric Encryption - answer-A cryptographic approach that uses a public key for encryption and a private key for decryption, improving security in communication. More suited to sensitive transactions of data compared to symmetric. Hashing - answer-A one-way function that transforms data into a fixed-length value called a hash. Commonly used for password storage and data integrity verification. A good hash function should minimise collisions and ensure small changes to an input create large changes to a hash value. Atomicity - answer-Guarantees that a transaction is treated as a single unit, either fully completing or having no effect at all.
Redundancy - answer-Unnecessary duplication of data within a database, which can lead to inefficiency and increased storage requirements. Referential Integrity - answer-Ensures consistency between linked tables by preventing invalid or broken relationships between records. Database Normalisation - answer-A process of organising a database to reduce redundancy and improve efficiency by dividing it into structured tables. First Normal Form (1NF) - answer-A table where each column contains atomic values (one value only), there are no repeating records or duplicate field name, and each row is uniquely identifiable by a primary key, ensuring structured and efficient data storage. Second Normal Form (2NF) - answer-Extends 1NF by ensuring that all non-key attributes are fully functionally dependent on the entire primary key. Third Normal Form (3NF) - answer-Further refines 2NF by removing transitive dependencies, ensuring all non-key attributes depend only on the primary key. Local Area Networks (LANs) - answer-A network that connects computers within a small geographic area, such as a school, home, or office. Wide Area Networks (WANs) - answer-A network that spans a large geographical area, often using third-party carriers to link multiple locations. Client-Server Network - answer-A network model where client devices request services from a central server that manages resources and processing. Central point of failure, knowledge, money and expertise needed to set up and maintain. Easy to manage updates, backups and security. Peer-to-Peer Network - answer-A decentralised network where devices communicate directly and share resources without a dedicated central server. Circuit Switching - answer-A communication method where a dedicated path is established between two nodes for the duration of the connection, e.g. traditional telephone networks; ensures consistent data flow but does not make the most efficient use of available bandwidth. Packet Switching - answer-A method of transmitting data by breaking it into packets that are sent independently over a network and reassembled at the destination, e.g. the internet; efficient for variable traffic but may introduce delays due to packet reordering. Domain Name System (DNS) - answer-A hierarchical system that translates human-readable domain names into IP addresses to locate network resources, e.g. 'google.com' to 142.250.190.78;
Protocol Layering - answer-A networking concept that divides communication functions into layers, each with specific protocols, makes it easier for manufacturer to develop hardware and software for specific layers. TCP/IP Stack - answer-A suite of protocols enabling reliable data transmission between networked computers, structured in four layers (Application, Transport, Internet, Link). Encryption - answer-The process of encoding data to prevent unauthorised access, ensuring only authorised recipients with the key can decrypt it. Firewalls - answer-Security barriers that monitor and control network traffic based on rules to protect systems from unauthorised access, e.g. blocking malware; enhances security but may slow traffic. Proxies - answer-A server that intercepts data packets to hide the source's network address, improving security and managing access, boosts privacy but can introduce latency. IP Address - answer-A unique numerical identifier assigned to a network. Used by routers to route data packets between networks; operates at the network layer, enabling communication across the internet. MAC Address - answer-A unique physical identifier hardcoded into a device's network interface card (NIC). Operates at the data link layer for local network communication, permanent but limited to LAN in scope. HyperText Markup Language (HTML) - answer-A markup language used to structure and display content on the web, e.g, using
tags for paragraphs; simple to use but limited to static content. Cascading Style Sheets (CSS) - answer-A styling language that controls the layout, appearance, and formatting of web pages, e.g. h1 { color: blue; } enhances design flexibility, can be written inline or external. JavaScript - answer-An interpreted programming language that enables interactive and dynamic content in browsers, e.g. form validation; adds interactivity but increases client-side processing load. Can be disabled and modified. Client-Side Processing - answer-Execution of scripts in the user's browser using local resources, e.g. JavaScript for animations; reduces server load but can be disabled or modified. Server-Side Processing - answer-Execution of scripts on a web server to process requests and return dynamic results, offers more control but increases server demand. Can be used to revalidate client-side processing.
Two's Complement - answer-A method representing negative numbers. Write the number out as a positive. From the right hand side, find the first bit that is a 1. Copy everything up to and including that 1. Flip all other bits to the left. MSB becomes negative. Sign and Magnitude - answer-A method of representing signed numbers in binary where the most significant bit indicates the sign (0 for positive, 1 for negative) and the remaining bits represent the number. Exponent - answer-The part of a number that specifies the number of times and direction to shift the floating point within the mantissa. Positive exponent, shift decimal to the right, negative, shift left. Allows large/small number representation but limited by bit size. Mantissa - answer-In floating-point representation, the fractional part of a number in normalised binary form. Normalisation in Binary - answer-A method to store real numbers in binary using a mantissa and exponent where normalisation adjusts the mantissa to start with 10 for negative numbers, 01 for positive, ensures unique representation and maximises precision. Floating Point Arithmetic - answer-Operations on decimal binary numbers. Using the exponent to calculate where the decimal place needs to move to within a sequence of a binary. Arrays - answer-A data structure storing a finite, ordered collection of same data type elements under one identifier, accessed by index. Fast access but fixed size. Contiguous meaning data exists together in memory. Lists - answer-A data structure maintaining a sequence of elements by index, dynamic in size and can hold multiple data types. Linked Lists - answer-A dynamic structure where nodes store data and a pointer to the next, easy to modify but slow traversal. Memory intensive due to use of pointers. Non contiguous meaning - answer-Data can exist anywhere in memory. Tuples - answer-An ordered, immutable collection of mixed data types, e.g. (1, 'hello', 3.14); stable but immutable. Records - answer-A structure grouping related fields into one unit, e.g. {name: 'Alice', age: 16}. Stacks - answer-A LIFO structure where the last item added is removed first. Queues - answer-A FIFO structure where the first item added is removed first.
Circular Queues - answer-A dynamic queue structure where the end connects to the beginning, reusing space via wrapping front and rear pointers. Trees - answer-A hierarchical structure of nodes with a root and children, e.g. file systems. Binary Search Tree - answer-A tree with at most two children per node, left smaller, right larger, fast to search, but data needs to be ordered. Graphs - answer-A structure of nodes and edges for relationships, e.g. social network relationships; versatile but memory-heavy. Directed Graphs - answer-A graph with one-way edges, e.g. A → B. Undirected Graphs - answer-A graph with bidirectional edges, e.g. A ↔ B. Breadth First Traversal - answer-A graph traversal exploring neighbors first, using a queue, e.g. level-by-level. Depth First Traversal - answer-A graph traversal going deep before backtracking, using a stack, e.g. branch exploration. Hash Table - answer-A structure mapping inputs to indexes via a hash function, fast retrieval but collision risk. Dynamic Data Structures - answer-A structure that grows or shrinks at runtime, e.g. linked lists; adaptable but complex to manage. Boolean Logic - answer-A branch of algebra where logical operators (AND, OR, NOT) are used to process true or false values, underpinning digital circuits. Boolean Expressions - answer-An expression composed of Boolean variables and logical operators that evaluates to either TRUE or FALSE, e.g. A ∧ B ∨ ¬C. Association Laws - answer-Rules stating that the grouping of terms does not affect the result: A ∧ (B ∧ C) = (A ∧ B) ∧ C and A ∨ (B ∨ C) = (A ∨ B) ∨ C, simplifying expressions. Commutation Laws - answer-Rules stating that the order of operands does not affect the result: A ∧ B = B ∧ A and A ∨ B = B ∨ A, showing symmetry in logic. Distribution Laws - answer-Rules defining how AND and OR operations distribute: A ∧ (B ∨ C) = (A ∧ B) ∨ (A ∧ C) and A ∨ (B ∧ C) = (A ∨ B) ∧ (A ∨ C), key for simplification. De Morgan's First Law - answer-The negation of a disjunction is the conjunction of the negations: ¬(A ∨ B) = ¬A ∧ ¬B, used in circuit design and proofs.
Intellectual Property Protection - answer-Establishes legal ownership rights for literary, dramatic, musical, and artistic works, including software, preventing unauthorised use. Copyright Duration - answer-Ensures copyright protection for 70 years after the creator's death or creation if anonymous, e.g. software protected until 2095 if created in 2025. Illegal Distribution - answer-Prevents the unauthorised copying or sharing of protected works, e.g. pirating software or music, with fines or imprisonment. Digital Copyright - answer-Applies copyright laws to digital content, including music, films, and software, e.g. protecting a downloaded game from being copied. Data Protection Act (2018 - UK GDPR) - answer-A UK law from 2018, aligned with GDPR, regulating personal data processing, storage, and access, e.g. requires consent for data collection; ensures privacy and user rights but imposes compliance costs on organisations. Regulation - answer-Determines how personal data is processed, stored, and accessed, aligning with GDPR to ensure lawful handling, e.g. consent for data collection. User Rights - answer-Introduces strict guidelines for users to access, modify, and delete their personal data, e.g. right to erasure or data portability. Security Requirements - answer-Mandates data controllers and processors to ensure data is protected, e.g. encryption or secure storage to prevent breaches. Accountability - answer-Requires organisations to justify and document data collection and processing practices, e.g. maintaining records for audits. Regulation of Investigatory Powers Act (2000) - answer-A UK law from 2000 allowing authorised surveillance and interception of communications for security, e.g. monitoring emails with warrants; enhances public safety but raises privacy concerns with oversight challenges. Surveillance Powers - answer-Grants government bodies the authority to monitor electronic communications with warrants, e.g. for national security. Interception of Communications - answer-Allows agencies to intercept phone calls, emails, and internet activity with proper authorisation, e.g. tracking terrorism suspects. Data Access - answer-Permits access to encrypted communications and stored data for national security purposes, e.g. requiring decryption keys from providers. Public Safety & Privacy - answer-Balances law enforcement capabilities with individual rights and privacy protections, e.g. oversight by judicial bodies.
Artificial Intelligence & Bias - answer-Examines ethical concerns in AI to ensure fairness, e.g. biased hiring algorithms misjudging candidates; promotes equitable decision-making but challenging to eliminate bias due to data or design flaws. Automation & Job Loss - answer-Assesses the social and economic impact of automation replacing human tasks, e.g. self-checkouts reducing cashier jobs; increases productivity but leads to workforce displacement and retraining needs. Privacy Issues - answer-Addresses ethical risks of data collection and consent in digital systems, e.g. tracking via cookies without user approval; safeguards personal autonomy but weak enforcement hinders protection. Environmental Impact - answer-Evaluates the ecological consequences of computing, e.g. e- waste from obsolete devices; encourages sustainable tech practices but high implementation costs delay progress. Digital Divide - answer-Explores social inequity in technology access, e.g. rural areas lacking broadband. Urban Connectivity - answer-Fosters inclusion through wider access but resource demands slow efforts. Monitor Behaviour - answer-Involves using computers to ethically track actions, e.g. CCTV for security; enhances public safety but risks privacy invasion if unregulated. Analyse Personal Information - answer-Processing personal data with ethical oversight, e.g. medical records improving healthcare; enhances service quality but risks misuse if data security fails. Piracy and Offensive Communications - answer-Addresses ethical and legal issues of illegal copying and harmful content, e.g. pirated software or hate speech; protects intellectual rights and social harmony but control is difficult online. Layout, Colour Paradigms and Character Sets - answer-Considers cultural influences on software design, e.g. red symbolising danger in the West, luck in China; improves user experience through cultural adaptation but increases design complexity. Abstraction - answer-Simplifying a problem by focusing on essential details and removing irrelevant ones, reduces complexity for design but may overlook context-specific details. Data Abstraction - answer-Representing data based on its logical behavior and operations, hiding implementation, e.g. stacks with push/pop.
Global Variables - answer-Variables declared globally, accessible anywhere, e.g. score = 0; simplifies access but risks unintended changes. Integrated Development Environment (IDE) - answer-A tool aiding coding with debugging and testing features, streamlines development. Iterations - answer-Repeating code until a condition or count is met, e.g. while loop; automates repetition but risks infinite loops. Local Variables - answer-Variables scoped to a subroutine, e.g. temp in a function; reduces conflicts but limits scope. Modularity - answer-Dividing a program into reusable modules, e.g. input/processing files; enhances maintenance but requires planning. Parameter Passing by Reference - answer-Passing a variable's address to modify it, e.g. altering a list; efficient but risks accidental overwriting of original variable. Parameter Passing by Value - answer-Passing a copy of a variable, safe but memory-intensive. Parameters - answer-Inputs to functions/procedures, e.g. x, y in multiply(x, y); customises behavior but requires correct passing. Procedures - answer-Code blocks performing tasks without return, e.g. def print_message(); organizes logic but no output. Recursion - answer-A function calling itself with a base case; elegant but memory intensive. Sequences - answer-Executing statements in order, e.g. print 'A' then 'B'. Subroutines - answer-Independent code performing tasks, e.g. calculate_total(). Backtracking - answer-Exploring paths and retreating from dead-ends, e.g. maze solving; finds solutions but slow for large problems. Computable Problems - answer-Problems solvable with finite steps, e.g. sorting; enables automation but excludes undecidable issues. Computational Methods - answer-Algorithmic techniques for problem-solving. Data Mining - answer-Extracting patterns from large datasets, e.g. sales trends; reveals insights but computationally intensive. Divide and Conquer - answer-Splitting problems into sub-problems, e.g. merge sort.
Heuristics - answer-Approximating solutions for complex problems, e.g. A* pathfinding; fast but not always optimal. Performance Modelling - answer-Simulating system behavior under conditions, e.g. network load; predicts performance, not always accurate. Problem Decomposition - answer-Breaking problems into manageable parts, e.g. game graphics/logic. Problem Recognition - answer-Identifying solution patterns, understanding common inputs, processes and outputs. Visualisation - answer-Using diagrams to model algorithms, e.g, flowcharts; aids understanding but time-consuming. A* Algorithm - answer-An informed search estimating shortest paths with distance and cost, e.g. game pathfinding; efficient but heuristic-dependent. A good enough approach to pathfinding. Big O Notation - answer-Classifying efficiency by worst-case time/space complexity. Describes the time and space complexity of a given algorithm based on the size of the input/dataset. Binary Search - answer-A logarithmic search dividing sorted lists, O(log n); fast but requires data to be sorted first. Bubble Sort - answer-Swapping adjacent elements until sorted, O(n²), simple but slow for large data sets. Dijkstra's Shortest Path Algorithm - answer-Finding shortest paths by exploring all options, e.g. network routing; optimal but slow for large graphs. Insertion Sort - answer-Building a sorted list incrementally, O(n²), intuitive but inefficient for data that has no order to it already. Linear Search - answer-Checking each item sequentially, O(n), simple but slow. Merge Sort - answer-Dividing and merging sublists, O(n log n), efficient but space-heavy. O(1) (Constant Time/Space) - answer-Fixed time/space regardless of input, e.g. array access; fastest but rare in algorithm design. O(2ⁿ) (Exponential Time/Space) - answer-Exponential growth with input, e.g. recursive Fibonacci; powerful but impractical. O(log(n)) (Logarithmic Time/Space) - answer-Logarithmic growth, e.g. binary search; scalable but needs structure.
