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11. lf paper_____, we would have no books.
A. was not invented
B.didn't invent
C.had not invented
D. had not been invented
12. You could rarely see him in the library,____you?
A.did
B.didn't
C.could
D.couldn't
13. I____harder, or [ would have passed the examination.
A.should have worked
B. must have worked
C.could work
D. might work
14. The man was charged for making his employees____ overtime without extra pay.
A. to work
B. working
C. work
D. worked
15. No one would know what happened last night if you could keep your mouth ______
A.shut
B. to shut
C.shutting
D. to be shut