第一节 单项填空（共15小题；每小题1分，满分15分）
从每题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21. In America, if ________ friend has invited you to drop by anytime, it is ________ best to call before visiting to make sure it is convenient for him.
A. a; a B. the; a C. a; 不填 D. the; 不填
22. Some people would rather ride bikes as bike riding has ________ of the trouble of taking buses.
A. nothing B. none C. some D. neither
