Researches are convinced the positive effect of laughter physical and mental health.
A. of; at
B. by; in
C. of; on
D. at; on
网友回答
A. of; at
B. by; in
(答案→)C. of; on
D. at; on
解析：考查介词：be convinced of相信…，第二空填on，effect on…对…的影响，句意：研究者相信，笑声对身体精神健康的积极影响。选C。