【单选题】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．they had been done
C．having been done
D．they were done
【单选题】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．they had been done
C．having been done
D．they were done
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