21. To me, cycling at ___ altitude of more than 4,000 meters was _____ unforgettable experience.
A. the; an B. an; an C. /; a D. an; /
22. ------- Jack, you erase the blackboard today, __________?
------- But I did it yesterday.
A. will you B. do you C. would you D. don’t you
23.Planning so far ahead______ no sense ----- so many things will have changed by next year.
A. made. B. is making C. has made D. makes
