7.Although the causes of cancer ____， we do not yet have any practical way to prevent it.  
A．a(chǎn)re being uncoveredB．have been uncovering
C．a(chǎn)re uncoveringD．have uncovered
A
分析語境邏輯不難發(fā)現(xiàn)，該空表示“現(xiàn)在正在被揭示”，應(yīng)填現(xiàn)在進行時被動語態(tài)動詞形式are being uncovered。
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