Which of the following describes the helpful role that bacteria play in nitrogen fixation?
"A. Bacteria convert atmospheric carbon dioxide into a form of trapped carbon that animals can use. B. Bacteria convert yogurt into cheese, which can be eaten as food by humans or animals. C. Bacteria convert a healthy animal into a sick animal by producing a toxin or growing into a large population.D. Bacteria convert atmospheric nitrogen into a nitrogen-containing ion that plants can absorb."