By the time some scientists the lost towns, they an awesome historical site.A. had found

By the time some scientists the lost towns, they an awesome historical site.
A. had found, had found
B. found, found
C. found, had found
D. had found, found

A. had found, had foundB. found, found(答案→)C. found, had foundD. had found, found解析：考查时态。By要和完成时连用，如果by后面的句子使用一般过去时，主句使用过去完成时的形式。句意：到一些科学家分析那些失踪的城镇的时候，他们已经发现了这些有历史意义的遗址。故C正确。

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