问题

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

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

正确答案是：

同类题型

The problem ______ many children,especially girls,have ______ school is getting more and more serious.

_____________ the doctor , the old man is out of danger.

_____ useful advice it is！

26个字母中能够单独构成单词的两个字母是 .

I don’t know _____ tomorrow. Can you tellme?