单项选择题

After people have learned that magnets attract things, centuries passed ______ they took note of the fact that magnets sometimes also repel things.

A. before
B. until
C. after
D. since
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单项选择题
Many farmers came to the city, ______ jobs and for all the other legendary opportunities of life in the city.
A. looking for
B. to look for
C. having looked for
D. looked for
单项选择题
Had he worked harder, he ______ the exams.
A. must have got through
B. would have got through
C. would get through
D. could get through
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