20.Nowadays, most people，in order to keep fit，prefer walking to driving ____necessary.

A. if　　 B. unless　　 C. in case　　 D. as if

19._____with this challenge, he should meet it without any hesitation.

A. Facing　　　 B. Having faced　　 C. Being faced　　 D. Faced

18. Only when we are abroad ____feel how important English is.

A. we will　　 B. do we　　 C. will we　　 D. are we

17. He has known the truth. Therefore, there is no ____ to keep it a secret

A. chance　 B. doubt　 C. possible　 D. likely