一辆汽车沿平直公路行驶，开始以20m/s的速度行驶了全程的1/4，接着以速度v行驶其余的3/4的路程，已知全程的平均速度为16m/s，则v等于
A．18m/s
B．36m/s
C．15m/s
D．17.1m/s
答案：C
题目分析：平均速度等于位移与时间的比值，根据题意
。
可以求出：
答案选C。
点评：根据平均速度的定义公式去求解速度v，是常规题，比较容易，考试时不能丢分．
| 根据对话内容及方框中所给的句子补全对话。其中有一项是多余的。 | |
| A: Excuse me, sir. 1 . B: Yes. What can I do for you, young man? A: 2 . Some of my classmates want to go to Beijing for a short visit. I'd like to look for some sightseeing information. I want to know if you have a four days' tour. B: We certainly do. There are quite many tours. 3 . A: Well, we are going to visit some famous places such as the Great Wall, the Summer Palace and so on. B: They are all places of great interest in Beijing. 4 . A: Well, today is June 24. And we still have three more days before finishing the final exam. 5 . B: Oh, I see. Well, there are two kinds of tours: package and self-service. A: I think we'II take the self-service. B: All right. Here is the information about the tour and it will help you. A: Thank you very much. B: You are welcome.
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