15.The book,_____the cover is broken,is not mine.

　　　 A.of it　 B. for which　 　　　 C. whose　　 　　 D. of　 which

14. We Chinese, ________ having made great achievements in our socialist constructions, still have a long way to go.

　　　 A. once　 B. when　　　　 C. unless　　　 D. though

13. Their newly-built school is almost ______ ours which was built in the 1910s.

　　　 A. the twice size of 　　 B. as twice large as　　　

　　　 C. larger twice than　　　 D. twice the size of

12.Not until now _____how serious the environment pollution is .

　　　 A. did we realize　　　　　 B. will we realize　

　　　 C. have we realized　　 D. had we realized