Hormone X Produces Its Effect in Its Target Cells Via

Hormone X produces its effect in its target cells via the cAMP second messenger system. Which of the following will produce the greatest effect in the cell?

A) a molecule of hormone X applied to the extracellular fluid surrounding the cell
B) a molecule of hormone X injected into the cytoplasm of the cell
C) a molecule of cAMP applied to the extracellular fluid surrounding the cell
D) a molecule of cAMP injected into the cytoplasm of the cell
E) a molecule of activated, cAMP-dependent protein kinase injected into the cytoplasm of the cell