题目内容：

They were surprised that a child should work out the problem ________ they themselves couldn't.
A.once B.then C.while D.if

优质解答

答案C 【巧解导析】此题是由两个分句组成的并列句．根据句意,分句之间表转折关系,四个选项中只有while是并列连词,含义为“而,却”． 查看原帖>>