第一节 单项选择（共20小题；每小题0.5分，满分10分）
请认真阅读下面各题，从题中所给的A、B、C、D四个选项中，选出最佳选项。
21. We are taking _______ flight to Morocco, after which we will travel through the Sahara Desert by ______ camel.
A. a; / B. /; a C. a; a D. /; /
22. Mountain climbing ______ be very tiring, and many people feel sick with the air ______ thinner.
A. may; gets B. may; getting C. can; gets D. can; getting
23. How did it ______ that you got lost in Downing Street last weekend?
A. come across B. take place C. occur D. come about
