Which statement correctly describes the role of nitrogen fixation in the nutrient cycle?
A) N2 is taken in by green plants and transformed into nitrogen-rich protein. B) N2 is inhaled by herbivores and transformed into ammonia, which can be dissolved in their blood. C) NH4+ is converted by certain bacteria into complex proteins that can then be ingested by animals. D) N2 is taken up by certain bacteria that can change it into chemically available forms usable by plants.