The best understood intracellular messenger is cyclic AMP (cAMP). Which of the following statement(s) concerning this intracellular messenger is/are correct? a. Intracellular cyclic AMP is constantly degraded by a specific enzyme, cAMP phosphodiesterase b. Most of the actions of cAMP are mediated by activation of protein kinase A c. Intracellular levels of cAMP are relatively stable and change solely in response to activation of adenylate cyclase d. cAMP is the only cyclic nucleotide active as an intracellular messenger