12. In the reoxidation of QH₂ by purified ubiquinone-cytochrome c reductase (Complex III) from heart muscle, the overall
stoichiometry of the reaction requires 2 mol of cytochrome e per mole of QH₂ because:
(A)cytochrome c is a one-electron acceptor, whereas QH₂ is a two-electron donor.
(B)cytochrome c is a two-electron acceptor, whereas QH₂ is a one-electron donor.
(C)cytochrome c is water soluble and operates between the inner and outer mitochondrial membranes
(D)heart muscle has a high rate of oxidative metabolism, and therefore requires twice as much cytochrome c as QH, for
electron transfer to proceed normally.

(E)two molecules of cytochrome c must first combine physically before they are catalytically active.