Answer:
C. Some bacteria.
Step-by-step explanation:
Firstly here, nitrates bacteria turn ammonia compounds into nitrites by combining them with oxygen.(Nitrites are chemicals with less oxygen in their molecules than nitrates).
Secondly, nitrate bacteria combine nitrites with more oxygen to form nitrates. By this sequence, nitrogen in proteins is changed into a form which can be absorbed by plant roots. The same series of bacteria also form nitrates out of the nitrogen-containing compounds in animal droppings and urine.
Recently fixed ammonia is then converted to biologically useful forms by specialized bacteria.