单选题 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.

【正确答案】 D

【答案解析】由第一段知研究者们focus on serotonin基于两个原因：一是serotonin与大部分的 hallucinogens结构相似，而不是全部，故排除选项B，第二个原因即是选项D。