They were surprised that a four-year-old boy should work out such a difficult problem the

问题补充：

They were surprised that a four-year-old boy should work out such a difficult problem they themselves couldn’t.

A. once

B. then

C. while

D. if

答案：

A. once

B. then

C. while

D. if

解析：while是并列连词,意为“而,却”,前后有对比的意味。