Nitrifying bacteria obtain their chemical energy from:

A) the reduction of NO₃− to NH₄⁺.
B) the formation of NH₄⁺ from N₂ gas.
C) the oxidation of NH₄⁺ to N₂.
D) the oxidation of NH₄⁺ to NO₃−.
E) the formation of NO₂− from NO₃−.