1．It ________ (take) us a long time ________ (finish) ________　 (make) a model plane yesterday.

25．We must find ________ who has taken the book out of the　 library.

24．I do not know anybody ________ here except you.

23．What does she do every day ________ a librarian?

22．He didn't understand. it ________ the teacher explained it again to him.

21．Have you ever travelled ________ a plane?

20．Can you think _______ other ways to encourage students to study hard?

19．He may be late. What's ________.

　　 He doesn't know the change of the place for the meeting.

18．Is he ________ here?

　　 He should have gone home hours ago.

17．I'm leaving here ________ Shanghai tonight.