问题
选择题
Scientists are convinced ____ the positive effect of laughter ____ physical and mental health. [ ]
A. of;at
B. by;in
C. of;on
D. on;at
答案
答案：C
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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
答案：C