20. When you read a book, you’d better make a mark _______ you have any questions.

　 A. at which　　　　　 B. at where　　　　　 C. the place where　　　　 D. where

19. --- Look at the timetable. Hurry up! Flight 4026 ________ off at 18:20.

　 --- Ok.

　 A. took　　　　　　　 B. takes　　 　　　　　C. will be taken　　　　 D. has taken

18. John received an invitation to dinner, and with his work _______, he gladly accepted it.

　 A. finished　　　　　 B. finishing　　　　　 C. having finished　　　 D. was finished