I.单项选择：从A、B、C、D四个选项中，选出一个最佳答案。（15分)
( )21. As soon as the teacher came in, the boys stopped ____.
A. laugh B. to laugh C. laughing D. laughed
( )22. Mr. Li makes me ____, because it is bad for my health.
A. don’t smoke B. not smoke C. not smoking D. not to smoke
( )23.Helen’s mother is__________Helen’s study ______she missed some classes.
A.bored with , because B.pleased with , because of
C.worried about , because of D.worried about , because
( )24.Kangkang_______be at home . He rang me from Harbin five minutes ago .
A. can’t B. mustn’t C.may not D.won’t
