According to the passage, which of the following is one of the primary factors that led researchers studying hallucinogenic drugs to focus on serotonin
A. The suppression of the activity of serotonin-secreting neurons by the administration of hallucinogens.
B．The observed similarities in the chemical structures of serotonin and hallucinogens.
C．The effects the administration of hallucinogens has on serotonin production in the human brain.
D．Serotonin-induced changes in the effects of hallucinogens on behavior.