Sutherland found evidence for the presence of a G protein- coupled receptor for epinephrine in muscle cells. For this receptor to function normally, it must have at least how many binding sites, and what must be bound?

a. One: G protein
b. Two; epinephrine and G protein
c. Two: epinephrine and GTP
d. One: extracellular epinephrine
e. Three: epinephrine, G protein, and ATP