Although the causes of cancer ____， we do not yet have any practical way to prevent it
A.are being uncoveredB.have been uncoveringC.are uncoveringD.have uncovered在线课程A
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。