3. When glucagon binds to its receptor on a liver cell, which of the following sequence of events occurs to activate protein kinase A?
(A) activation of adenylate cyclase, activation of G protein, production of cAMP
(B) activation of G protein, activation of adenylate cyclase, production of cAMP
(C) activation of G protein, production of cAMP, activation of adenylate cyclase
(D) production of cAMP, activation of adenylate cyclase, activation of G protein
(E) none of the above