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
