The millions of circulations 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

The millions of circulations 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

出自：安徽理工大学-电气工程及其自动化

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