They were surprised that a child should work out the problem________they themselves couldn't．
A.onceB.thenC.whileD.if在线课程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。