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

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：考查被动语态。选项A是现在进行时的被动；选项B是现在完成进行时的主动语态；选项C是现在进行时的主动语态；选项D是现在完成时的主动语态。根据题意：虽然癌症的成因长在被揭示，但我们眼下还没有防止它的实用方法。故答案为A.