Why is bacteria an essential part of the nitrogen cycle?

Bacteria use the nitrogen which prevents harm to other organisms.

Bacteria use the nitrogen that has been fixed by plants to create atmospheric nitrogen.

Bacteria are responsible for fixing nitrogen into usable forms for other organisms.

Bacteria, through assimilation, absorb nitrates or ammonium ions from the soil.

I'm not sure if it is B, which is what I would think.

Question

Why is bacteria an essential part of the nitrogen cycle?

Bacteria use the nitrogen which prevents harm to other organisms.

Bacteria use the nitrogen that has been fixed by plants to create atmospheric nitrogen.

Bacteria are responsible for fixing nitrogen into usable forms for other organisms.

Bacteria, through assimilation, absorb nitrates or ammonium ions from the soil. 

I'm not sure if it is B, which is what I would think.

anonymous · Answer

C.  diazotrophs fix nitrogen. 
A makes no sense at all. 
B is wrong as Plants do not fix nitrogen.
D is wrong as assimilation is when the nitrates are incorporated into plants.