摘要： Scientists are convinced the positive effect of laughter physical and mental health . A．of, at B．by ,in C．of ,on D. on ,at
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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 |
Scientists are convinced the positive effect of laughter_____physical and mental health.
| A．of, at | B．by, in | C．of, on | D．on, at |
Scientists are convinced _____ the positive effect of laughter _____ physical and mental health.
| A．of; at | B．by; in | C．of; on | D．on; at |