had been puzzled over the problem for over an hour without any result
问题描述：
had been puzzled over the problem for over an hour without any result
I had been puzzled over the problem for over an hour without any result,________ all at once the solution flashed across my mind.
A while
B when
C then
D as
应该选B.为什么?各个选项都讲一遍为什么不对.
答
我已经对这个难题思考了一个小时还是没有结果,正在这时答案有我的头脑中显现!when翻译为正在这时（有突然发生的意思）,这是它的特殊用法!while ,as充当时间状语时只能表示一边一边故代入句子明显不和理!then那时代入...