Althoughthecausesofcancer

問題詳情：

 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

【回答】

知識點：現在進行時及其被動式

題型：選擇題