They were surprised that a child should work out the problem they themselves couldn’t.A.

问题补充：

They were surprised that a child should work out the problem they themselves couldn’t.

A. when

B. where

C. until

D. while

答案：

A. when

B. where

C. until

(答案→)D. while

解析：考查连词辨析。while是个并列连词，表示“转折”，而其它连词均没有转折之意。句意：他们很惊讶一个孩子能够解决出这个问题，而他们不能够。