Scientists are convinced___the positive effect of laughter___physical and mental health.（）
| A． | of;at | B． | by;in |
| C． | of;on | D． | on;at |
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Scientists are convinced___the positive effect of laughter___physical and mental health.（）
| A． | of;at | B． | by;in |
| C． | of;on | D． | on;at |
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