They kept trying they must have known it was hopeless.
A. if
B. because
C. when
D. where
网友回答
A. if
B. because
C. when
D. where
解析：
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。