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
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单项选择题
A large fish was slowly swimming through the water, its tai ______ back and forth like the pendulum of a clock.
A．swung
B．was swinging
C．was swung
D．swinging
单项选择题
Televisions enable us to see things happen almost at the exact moment ______.
A．which they are happening
B．they are happening
C．which they happen
D．they have happened
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