The enzyme pyruvate kinase catalyzes the conversion of phosphoenolpyruvate to pyruvate; the phosphate group is transferred to ADP to form ATP. This reaction is an example of

A) ATP synthesis by substrate-level phosphorylation.
B) the use of a proton concentration gradient by [H+]-ATPases to synthesize ATP.
C) ATP coupled to FADH2 oxidation.
D) a P-Type ATPase activity.
E) oxidative phosphorylation.