问题
选择题
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
答案
答案：B
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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
答案：B
