They kept trying they must have known it was hopeless.A. ifB. becauseC. whenD. where

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A. if
B. because
(答案→)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。