20. The allosteric enzyme PFK-1 (Phosphofructokinase-1) catalyzes the committed step of glycolysis. Which of the following statements about PFK-1 is correct? (A) It is inhibited by ADP and stimulated by AMP. (B) It is inhibited by high pH. (C) It is stimulated by citrate. (D) It is stimulated by insulin but is inhibited by epinephrine in muscle cells. (E) It is stimulated by insulin and inhibited by glucagon in liver cells.