Some Hormones Act to Regulate the Concentration of Intracellular Signals

Some hormones act to regulate the concentration of intracellular signals like cyclic adenosine monophosphate (cyclic AMP, or cAMP) by activating or inhibiting adenylyl cyclase, the enzyme responsible for its production, or by activating or inhibiting phosphodiesterases, which are responsible for its destruction. Cyclic AMP, in turn, functions by activating enzyme cascades that set certain metabolic pathways in motion. The regulation of cAMP levels in cells is an example of

A) homeostasis.
B) rheostasis.
C) equilibrium.
D) adaptation.
E) all of these.