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单选题
For years, doctors ________ millions of patients＇ lives with the help of microscopes.
A
have saved
B
are saving
C
will save
D
were saving
答案解析
正确答案：A
解析：
参考解析:现在完成时。状语是"多年来”，指从过去某一时刻开始起一直持续到现在，是典型的用来指导完成时的标志。A项是过去完成时，其他都不是。B项是现在进行时，表示正在进行的动作，不符合。C项是将来时。D项是过去进行时，表示过去某一时刻正在进行的动作，这三项都不符合。句意:多年以来，医生们已经借助于显微镜挽救了数百万人的生命。
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