题目内容：

the fire lasted for 20 days,(during that time)the whole forest was destroyed.
为什么不能填during which time

优质解答

如果用which,那应该是during which,即the fire lasted for 20 days,during which the whole forest was destroyed.这句是非谓语从句,which 就整个指代了for 20 days.如用during which time,那么这个which即与time重复,或许你可以这么理解.