18. cAMP and cGMP are ____________________ with phosphate esterified as a cyclic ____________________ and are important as ____________________ of cellular metabolism. (A) nucleotides； phosphodiester； inhibitors, (B) nucleotides； phosphomonoesters, regulators, (C) nucleotides； phosphodiesters, regulators, (D) nucleosides； phosphomonoesters, stimulators, (E) all of the above.