Through which process is nitrogen gas converted to ammonia? 

A)  nitrogen fixation 
B)  nitrification 
C)  denitrification 
D)  assimilation

Nitrogen gas is converted into ammonia through a process called nitrogen fixation. This process is carried out by certain types of bacteria, which convert nitrogen gas into ammonia that can be utilized by plants and other organisms. Nitrification involves the conversion of ammonia into nitrites and then nitrates, while denitrification converts nitrate back into nitrogen gas. Assimilation involves the incorporation of nitrogen into organic molecules.