Which Observation Suggested to Sutherland the Involvement of a Second

Which observation suggested to Sutherland the involvement of a second messenger in epinephrine's effect on liver cells?

A) Receptor studies indicated that epinephrine was a ligand.
B) Glycogen breakdown was observed only when epinephrine was administered to intact cells.
C) Glycogen breakdown was observed when epinephrine and glycogen phosphorylase were combined in a cell-free system.
D) Epinephrine was known to have different effects on many types of cells.