Which of the following observations suggests that a second messenger is required for the activation of glycogen phosphorylase by epinephrine?

A) Glycogen phosphorylase is inactive in the absence of epinephrine in a cell-free system.
B) Glycogen breakdown occurs only when epinephrine is administered to intact cells.
C) Glycogen breakdown occurs when epinephrine and glycogen phosphorylase are combined in a cell-free system.
D) Epinephrine binds directly to glycogen phosphorylase.