问题
选择题
Scientists are convinced _____the positive effect of laughter _____physical and mental health．
A．of ;on
B．by; in
C．of ; at
D．on ;at
答案
答案：A
考查固定短语。be convinced of确信; effect与介词on连用。句意：科学家相信笑对于身体和精神的积极影响。
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