I had been puzzled over the problem for over an hour without any result all at once a sol

问题补充：

I had been puzzled over the problem for over an hour without any result, all at once a solution flashed across my mind.

A. while

B. when

C. then

D. as

答案：

A. while

(答案→)B. when

C. then

D. as

解析：考查连词的用法。while当什么的时候，when正在那时，then那时，as意为，当什么的时候，句意：对这个问题我已经困惑了一个多小时了没有任何的结果，突然正在那时一个解决方案在我脑海闪现。故选B。