21. Which of the following statements about insulin is correct?
(A) Insulin binds to its receptor on liver and muscle cells, which activates a signaling cascade thatleads to the activation of glucose transporters (GLUT4) and enzymes involved in glycogensynthesis
(B) Insulin activates glycogen phosphorylase, which breaks down glycogen into glucose andpromotes its uptake by cells
(C) Insulin inhibits the activity of glucagon, which promotes glycogen synthesis and glucose uptakein liver and muscle cells
(D) Insulin binds to its receptor on adipose cells, which activates a signaling cascade that leads to theuptake of glucose and its storage as glycogen in liver and muscle cells