How Is Atmospheric Nitrogen Converted to a Usable Form in the Biosphere

Multiple Choice

How is atmospheric nitrogen converted to a usable form in the biosphere?

A) Atmospheric nitrogen is directly available to plants and animals.
B) It is absorbed into plants via stomata and converted within the plant to a usable form.
C) Bacteria within root nodules of legumes chemically combine the atmospheric nitrogen into nitrates and ammonia.
D) Atmospheric nitrogen is completely unavailable to plants and animals and, as a result, synthetic fertilizers must be used.