Hormone X activates the cAMP second messenger system in its target cells. The greatest response by a 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 cAMP into the cytoplasm of the cell.
E) injecting a molecule of activated, cAMP-dependent protein kinase into the cytoplasm of the cell.