Why Is Nitrogen Fixation Such an Important Process

Why is nitrogen fixation such an important process?

A) Nitrogen fixation can only be done by certain prokaryotes.
B) Fixed nitrogen is most often the limiting factor in plant growth.
C) Nitrogen fixation is very expensive in terms of metabolic energy.
D) Nitrogen fixers are sometimes symbiotic with legumes.
E) Nitrogen-fixing capacity can be genetically engineered.