IV．单项选择（共10小题，计10分）
本题共有十个小题，请从每个小题的四个选项中，选出一个最佳答案
21. I think ________ is our duty to keep our classroom clean and tidy.
A. that B. this C. it D. one
22. Before I started high school, I used _________ a lot of time_______ games with my friends.
A. to spend; to play B. spend; spending C. spend; play D. to spend; playing
23. All my family are very busy.________ my mother________my sister watches TV plays these days.
A. Either; or B. Both; and C. Neither; nor D. Neither; or
