As part of the nitrogen cycle, bacteria convert certain nitrogen-containing molecules from one type to another. Which stage of the nitrogen cycle does the arrow in the image represent?
A. Nitrogen fixation, in which nitrogen is sunk into the terrestrial reservoir.
B. Nitrification, in which nitrogen is sunk into the atmosphere.
C. Denitrification, in which nitrogen is sunk into the atmosphere.
D. Ammonification, in which nitrogen is sunk into the terrestrial reservoir.