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487.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
488.Oh, God! What did you do to my report, Tom?
“Terribly sorry. My mind _____ and I will type it again for you.
A.has wandered
B.had wandered
C.is wandering
D.was wandering
