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

问题补充：

Scientists are convinced the positive effect of laughter physical and mental health.

A. of; at

B. by; in

C. of; on

D. on; at

答案：

A. of; at

B. by; in

C. of; on

D. on; at

解析：be convinced of 确信。effect on……，有关于------方面的影响。