15.　 D。固定短语stop sb. from doing，本句意为“阻止交通事故的发生。

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Bob: In a book I was reading, some men had to 　1　 　a desert. There was nothing 　2　 　sand they could see. It was so large 　3　 　it seemed to them that it had almost no 　4　 .

Tom: It 　5　 　be very hot. Bob: That’s right. 　6　 　hot in a desert. But they rested 　7　 　the day　　　　　　　　　　　　　　　 and traveled at night.　　　　　　　　　　　　 .　　　　　　　　　　　　　　　　

Tom: But there’re no trees or anything like that 　8　 　you to get cool.

Bob: 　9　 . But they had some tents (帐篷). They 　10　 　when they were going to rest.

Tom: That was a lot of trouble, 　11　 ?

Bob: Yes. Every night 　12　 　they 　13　 　on the next part of the trip, they had to take the tents 　14　 . Then they had to put them on their camels. That’s the 　15　 　way you can travel in a desert.

14.　 B。建议人们应经常使用立交桥，其它选项在意思上都不对。

13.　 A。两者都(老年人和年轻人)。

12.　 D。指过去没有立交桥时，既有人又有车辆过马路，with“伴有”，“带有”。

11.　 A。 老人上下立交桥当然是有点困难，difficult形容词作宾语补足语。

10.　 D。建立交桥一方面有利安全，另一方面帮助人们。所以人们应该使用它们，而不是横穿马路(有如在建立交桥之前)。instead of “而不是”，“代替”。

9.　　　 B。spend…(in) doing　 意为“在建立交桥上花费了大量的财力”。

8.　　　 C。keep … doing 意为“让……一直干某事” 。

7.　　　 C。建造立交桥.。

6.　　　 B。why引导表语从句，意为“这就是……的原因”。