67.
A:
Cholera toxin activates a Gi (inhibitory) protein.
B:
Cholera toxin inhibits phosphodiesterase so that the signal is not switched off.
C:
Cholera toxin inhibits the binding of vasoactive intestinal polypeptide to the receptor.
D:
Cholera toxin inhibits the GTPase activity of the G protein alpha subunit.
The answer is: D