第一节 单项填空 （共10小题；每小题1分，满分10分）
从A、B、C、D选项中，选出可以填入空白处的最佳选项，并填写在答题卡上。
21. Some people argue using ______ nature to meet our own needs may spell disaster, unless we start to look around and understand how ______ clean environment benefits us all.
A. /; a B. the; the C. the; a D. a; a
22. While watching television, _____________.
A. the doorbell rang B. the doorbell rings
C. we heard the doorbell ring D. we heard the doorbell rings
