二、单项填空 （本大题共15分，每小题1分）
在A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
( )21. It ________ over eleven years since the cat ____________.
A. is; died B. is; has been dead C. has been; has died D. was; has died
( )22. My sister, Lucy, has worked for a long time. He ____________ be very tired.
A. must B. can’t C. could D. mustn’t
( )23. ---I’m going to Shanghai for a holiday next weekend.
--- ___ you are there, could you buy me some green tea?
A. Because B. While C. During D. As
