单项选择题

Unless ______ by credit card, please pay in cash.

A. pay
B. is paid
C. paying
D. paid
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单项选择题
The millions of calculations involved, had they been done by hand, ______ all practical value by the time they were finished.
A. had lost
B. would lose
C. would have lost
D. should have lost
单项选择题
Good travelers don't leave ______ on trains or airplanes.
A. rubbish
B. litter
C. garbage
D. pollutant
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