In the Reoxidation of QH₂ by Purified Ubiquinone-Cytochrome C Reductase

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 c 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 QH₂ for electron transfer to proceed normally.
E) two molecules of cytochrome c must first combine physically before they are catalytically active.