Scientists are convinced the positive effect of laughter physical and mental health.
A. of；at
B. by；in
C. of；on
D. on；at
网友回答
A. of；at
B. by；in
(答案→)C. of；on
D. on；at
解析：考查固定词组。固定词组be convinced of相信…；have an effect on…对…有影响；句意：科学家们相信微笑对于身体很精神健康都有积极的影响。故C正确。