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