77. (1) release of acetyl-CoA； (2) release of CO2； (3) release of NADH. Pyruvate dehydrogenase can convert pyruvate into acetyl-CoA, NADH, and CO2. What is its reaction order of preceding three steps? (A) (1) → (2) → (3) (B) (1) → (3) → (2) (C) (2) → (1) → (3) (D) (2) → (3) → (1) (E) (3) → (1) → (2)