Although the causes of cancer , we do not yet have any practical way to prevent it.A. are

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(答案→)A. are being uncovered
B. have been uncovering
C. are uncovering
D. have uncovered
解析：正确答案A。此题考查状语从句中时态。主语causes of cancer和谓语动词uncover存在动宾关系，故选用被动语态，排除选项B、C、D;致癌原因正被揭开，故选进行时的被动语态。
　　现在进行时表示现在正在进行的动作或现阶段一直在进行的动作，但说话时不一定正在进行，较一般现在时相比具有暂时性。