单项选择题

It was only after some progress ______ in the use and development of electric current that men began to realize the importance and possibilities of magnetism.

A. had been made
B. would have been made
C. has been made
D. was made
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单项选择题
The millions of calculations involved, ______ by hand, would have lost all practical value by the time they were finished.
A. had they been done
B. having been done
C. they had been done
D. they were done
单项选择题
What does the speaker think future advertisers should do
A．Be more careful about what they advertise.
B．Spend less money on advertising.
C．Advertise more for their products.
D．Use new advertising techniques.
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