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

问题补充：

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

A. are being uncovered

B. have been uncovering

C. are uncovering

D. have uncovered

答案：

(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。