【题目】Scientists are convinced ___________ the positive effect of laughter ___________ physical and mental health.
A. of; at B. by; in C. of; on D. on; at
在线课程【答案】C
【解析】
试题分析：考查固定词组。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。固定词组be convinced of相信…；have an effect on…对…有影响；故C正确。