问题

Scientists are convinced _ the positive effect of laughter __ physical and mental health.

A:of on
B:by in
C:of at
D:on at

正确答案是：

同类题型

What did you do on April Fool’s Day — I played a trick _______ my brother.

We _____others’ homework.

The play had already been on for quite some time when we _____ at the New Theatre[ ]

What can you see in these pictures

Would you please tell me________________