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