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：考察介词。Be convinced of 为固定词组，意为对……信服 effect on sth 意为对……的影响 为固定短语 ，所以选择C
句意为，科学家们对笑声对身心健康产生的影响深信不疑。