Scientists are convinced the positive effect of laughter physical and mental health．
A. of; on
B. by; in
C. of; at
D. on; at
网友回答
(答案→)A. of; on
B. by; in
C. of; at
D. on; at
解析：考查固定搭配。Be convinced of相信…；对…信服；第二空前面的effect要和介词on连用；句意：科学家们相信大笑对身体和精神都有积极的影响。故A正确。