20. ------Excuse me, Sir , shall all the students repeat the exam.

　 ------________. Only those who failed.

A. That’s right .　　　 B. I’m afraid not .　 C. Yes, of course.　 D. I’m not sure.

19. ------Make sure you have turned off the air conditioners before going out.

------________________.

　　　 A. Heard it　　　 B. Made it　　　 C. Forgot it　　　 D. Got it

18.　 He was about to tell me the secret ＿＿ someone patted him on the shoulder.

　　　 A. as　　　 B. until　　　　 C. while　　　　 D. when

17.　 He is the only one of the students who ＿＿ a winner of three years.

　　　 A. is　　　 B. are　　　 C. have been　　　 D. has