18. Anesthetics block pain by blocking the transmission of nerve signals. Which of the following chemicals might work as anesthetics?
(A) a chemical that opens thechannels at nerve terminals
(B) a chemical that inhibits the enzymes that degrade neurotransmitters
(C) a chemical that stimulates the free nerve endings of nociceptor afferent fibers
(D) a chemical that prevents the opening of voltage-gated Na* channels in membrane
(E) a chemical that inhibits the reuptake of neurotransmitters by transporters