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 .

A. once

B. then

C. While

D. if

答案：

A. once

B. then

(答案→)C. While

D. if

解析：

while（implying a contrast）含相对之意，表示对比转折。