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

A$)$ act as $C{a}^{2+}$ pumps, which generate a $C{a}^{2+}$ gradient.

B$)$ transfer electrons from $NA{D}^{+}$to $O_2.$

C$)$ act as proton pumps, which generate a proton gradient.

D$)$ transfer electrons from FAD to $O_2.$

E$)$ act as electron pumps, which generate an electron gradient.