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

 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

试题答案

解析：

A．时态，语态。癌症的原因是被发现，必须用被动语态。