In Electron Transport, the Three Protein Complexes (I, III, and IV)

In electron transport, the three protein complexes (I, III, and IV)

A) act as proton pumps, which generate a proton gradient.
B) transfer electrons from NAD+ to O₂.
C) transfer electrons from FAD to O₂.
D) act as electron pumps, which generate an electron gradient.
E) act as Ca²⁺ pumps, which generate a Ca²⁺ gradient.