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。