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 |
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