22. Within the citric acid cycle (Krebs cycle), what is the direct role of acetyl-CoA? (A) It acts as the final electron acceptor in the electron transport chain. (B) It directly catalyzes ATP formation via substrate-level phosphorylation. (C) It serves as the initial substrate that combines with oxaloacetate to initiate the citric acid cycle. (D) It regulates the activity of key enzymes within the citric acid cycle through allosteric modulation. (E) It transports protons across the inner mitochondrial membrane to generate the proton motive force.