问题
选择题
People taking the therapy learn what scientists _____ for years-laughter is the best medicine. [ ]
A. know
B. knew
C. have known
D. had known
答案
答案：C
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People taking the therapy learn what scientists _____ for years-laughter is the best medicine. [ ]
A. know
B. knew
C. have known
D. had known
答案：C