【正确答案】 C

【答案解析】[解析] 语义理解题。根据diminish所在句子The reason why laughter can reduce pain seems to be that it helps to produce a kind of chemicals in the brain which diminish both stress and pain笑能减少疼痛的原因似乎是因为笑有助于大脑产生某种能够减轻压力及疼痛的化学物质，可知diminish应与前面的 reduce同义。