五道英语题,望能说明原因?

五道英语题,望能说明原因?
21. The 2010 Shanghai World Expo is intended to achieve our common goal: ________world that is safer, cleaner and healthier than ________ one we’re liv ing in.A. the; the                 B. a; /                C. a ; the                  D. the ; /   28. --- Did you often e-mail your family during the first year at university?   --- No. I was so busy meeting new people that I didn’t write to my family as I _____.    A. ought to           B. should do             C. should have written   D. should have30. Mr. Fox fell ill, thus __________ it necessary for us to find another person for his job.A. having made              B. made                  C. making               D. to make31. Further space exploration has been _______ with the rapid development of China’s science and technology.A. made it possible                                      B. made possible          C. made it possibly                                            D. made possibly 35. --- I’m thinking of the test tomorrow. I’m afraid I can’t pass this time.   --- _______! I’m sure you’ll make it.A. Go ahead                    B. Good luck                    C. No problem                 D. Cheer up