单项选择题
Silk（）by Chinese for thousands of years now.
A.has used
B.has been used
C.was used
D.is used
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By the end of this year, I（）enough money for a holiday
A.will save
B.will be saving
C.will have saved
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By next year he（）in New York for five years.
A.has worked
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The work（）by the time you get there.
A.will have been done
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D.has done
