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

问题补充：

7.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：分析语境逻辑不难发现，该空表示“现在正在被揭示”，应填现在进行时被动语态动词形式are being uncovered。