基本信息
单项选择（共10小题，每小题1分，满分10分）
21. It makes difference whether we can have better understanding of the environment.
A. / ; a B. a ; / C. a ; a D. the ; the
22. that air conditioners cause so much pollution, I would never have bought one.
A. If I knew B. If I should know C. Had I known D. Were I to know
