Much labour would have been saved if the electronic computers_________ before.
A. had invented
B. were invented
C. should have been invented
D. had been invented
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Five minutes earlier, and we ____________ the last bus.
A. had caught
B. should have caught
C. could have caught
D. caught
But for friction, people _________ neither walk, nor even stand up.
A. can
B. were able
C. will be able
D. could
He is working hard for fear that he ________ .
A. fell behind
B. may fall behind
C. should fall behind
D. would fall behind
If they _______ the contract ahead of time, they could not have missed the plan.
A. signed
B. had signed
C. would have signed
D. should sign
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