21. He went to Beijing and returned ____ three days.

A. in             B. before               C. later                  D. after

20. He will come ____ three days.                    

A.before      B.after       C. in          D. later

19.The PLA man saved the boy ____ death.       

A. of      B.from        C.to         D.on

18.The group is made up____five students. And they are studying hard to make up____ the lost time.

A.of,of            B.for,for                C.of,for                 D.for,of

17.The boy is waiting ____ his sister and they will go to the hospital to wait ____ their sick mother.

A.for,on          B.on,for                C.for,for                D.on.on

16. Fresh air is good ____ your health.       

A.at             B.for             C.of        D.to

15. It's very kind____ you to repair the bike ____ me.

A.for.for        B.of,of                  C.of,for          D.for,of

14. We go to school ____ a bike.        

A.in       B.on         C.by            D.ove

13. It happened ____ the Long March.        

A. during      B.in        C.at         D.for

12. The window is never opened ____ in summer.

A. but   B. except C. except for   D. but for