Scientists are convinced___the positive effect of laughter _

Scientists are convinced___the positive effect of laughter ____physical and mental health.
A.of,at B.of,on C.on ,at D.by,in

选B.
be convinced of sth 意为 对某事确信
effect on sth 是 对某物产生的效果
句子的意思是：科学家确信笑会对人的身体和精神健康产生积极效果.