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：句意：科学家确信笑会对人的身体和精神健康产生积极效果。be convinced of sth 意为对某事确信；effect on sth. 是对某物产生的效果