Which observation suggested to Sutherland the involvement of a second messenger in epinephrine's effect on liver cells?a. Enzymatic activity was proportional to the amount of calcium added to a cell-free extract.b. Receptor studies indicated that epinephrine was a ligand.c. Glycogen breakdown was observed only when epinephrine was administered to intact cells.d. Glycogen breakdown was observed only when epinephrine and glycogen phosphorylase were mixed.