They were surprised that a child should work out the problem________they themselves couldn't．

A.once

B.then

C.while

D.if

答案及解析： C

解析：

由语境可知，两个分句的意义表示转折对比，故只能选择while，其余各项或者句意不通或者结构错误。题意：令他们吃惊的是，一个孩子能够做出的题目，而他们自己竟做不出来。