27. -----David has been in hospital for 3 days.

------Really? I ___ why he didn’t attend the meeting.

A. was wondering　　　　　　　 B. am wondering

C. wonder　　　　　　　　　　 D. had wondered

26. The serious situation requires the government ____ immediate measures to live through the economic crisis.

A. will take　　 B. has taken　 C. took　 D. take

25. ----The hurricane was indeed a disaster this year.

-----But________. Everything has returned to normal.

A. it was a nightmare　　　　 B. we managed somehow

C. that was a good point　　　 D. it was a terrible experience

24. Look at the timetable. Hurry up! Flight 4028 ____ off at 18:20.

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

23. The house belongs to my aunt but she ____ here since it was bought.

A. hasn’t lived　 B. didn’t live　 C. hadn’t lived　 D. doesn’t live

22. The colorful lights on the teaching building ____ the festive atmosphere.

A. add up to　 B. are added to　　 C. add to　 D. are added up to

21. It will make ____ big difference whether the opening ceremony becomes ____ success or not.

A. /; /　　　　 B. a; a　　　 C. a; the　　　　 D. a; /

21.(本小题满分14分)已知点()都在直线上，是直线l与x轴的交点，数列是公差为1的等差数列.

(1)求数列的通项公式.

(2)若,问是否存在,使得成立，若存在，求出k的值，若不存在，请说明理由.

(3)求证：.