11.He was busily engaged ______ writing letters.

　　A. on

　　B. in

　　C. with

　　D. by

　　12.Do remember to ______ the light when you leave the room.

　　A. turn up

　　B. turn on

　　C. turn off

　　D. turn down

　　13.The atmosphere consists of an ocean of gases ______ miles high.

　　A. hundreds

　　B. hundreds of

　　C. of hundreds

　　D. of hundreds of

　　14.Not only the students but also the old teacher ______ been changed.

　　A. was

　　B. were

　　C. has

　　D. have

　　15.—— ________ I had!

　　—— You really suffered a lot.

　　A. What a time

　　B. What time

　　C. How a time

　　D. how time