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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

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

 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. on; at            C. by; in            D. of; on

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