Scientists are convinced the positive effect of laughter physical and mental health.A. of; at B. by;

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【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

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【答案】C

【解析】be convinced of确信; effect on…对 ... 的作用。句意：科学家们相信笑对身心健康有积极作用。根据句意选C。