Which of the following statements concerning the action of G proteins on adenylate cyclase is true?
I. G proteins always serve to activate adenylate cyclase.
II. G proteins always serve to inhibit adenylate cyclase.
III. Some G proteins activate adenylate cyclase, while others inhibit it.
IV. G proteins do not act on adenylate cyclase at all.
A) I only
B) II, III
C) I, III
D) II only
E) III only
Answer: E) III only
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