问题
                    
Researches are convinced ______ the positive effect of laughter ______ physical and mental health
                            A:of; at
B:by; in
C:of; on
D:at; on
                        
                        B:by; in
C:of; on
D:at; on
正确答案是:
                         
                        
                        
                    同类题型
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