They kept trying ----- they must have known it was hopeless.

答案C when在此的意思不是“当……的时候”,而是“尽管”、“虽然”的意思。又如：He walks when he might take a taxi。尽管他可以坐出租车,但他却走路。 He stopped trying when he might have succeeded next time。尽管他本来下次就可以成功的了,但他却停止努力了。 The boy was restless when he should have listened to the teacher carefully。这男孩子本来应该专心听老师讲的,但他却坐立不安。 有许多同学只知道when表示“当……的时候”,而不知道它还有其他许多意思,除上面提到的表示“尽管”、“虽然”外,when还可表示“既然”、“考虑到”。如：Why do you want a new job when you've got such a good one already?既然你有了这么好的一份工作,你为什么还要找新的工作呢? 查看原帖