21. We work together to achieve our common purpose： world that is safer, cleaner and healthier than one we found．
A. the；the B. a；不填 `C. a；the D. the；不填
22. The young man，who by then admission to university，decided to do some part-time jobs to gain more practical experience．
A. gained B. was gaining C. has gained D. had gained
23. He removed the tree ____________ it was the best in the ground.
A. when B. if C. because D. as
24. ___________ I am willing to go, I would like it better if you went.
A. How B. Where C. While D. If
