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

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

1. A.
   while
2. B.
   when
3. C.
   then
4. D.
   as

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

　　A.while 　　 B.when 　　  C.then 　　   D.as

 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

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

　　A.while 　　B.when 　　C.then 　　D.as