12. Hormone X activates the cAMP second messenger system in its target cells. The greatest response by a target cell would come from _______.
(A) applying a molecule of hormone X to the extracellular fluid surrounding the cell
(B) injecting a molecule of hormone X into the cytoplasm of the cell
(C) applying a molecule of cAMP to the extracellular fluid surrounding the cell
(D) injecting a molecule of activated, cAMP-dependent protein kinase into the cytoplasm of the cell