题干

如图所示,在京昆高速公路266km处安装了一台500万像素的固定雷达测速仪,可以精准抓拍车辆超速,以及测量运动过程中车辆的加速度.若B为测速仪,A为汽车,两者相距355m,此时刻B发出超声波,同时A由于紧急情况而刹车,当B接收到反射回来的超声波信号时,A恰好停止,且此时A、B相距335m,已知声速为340m/s,则汽车刹车前的正常行驶速度大小为(   )

A:30m/s

B:20m/s

C:10m/s

D:15m/s

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答案(点此获取答案解析)

B

同类题5

阅读下列短文,根据短文内容,从短文后各题所给的A、B、C、D四个选项中,选择最佳选项。

Some people think that they have an answer to the problems of automobile crowding and pollution in large cities. Their answer is the bicycle, or bike.

In a great many cities, hundreds of people ride bicycles to work every day. In New York City, some bike riders have even formed a group called “Bike for a Better City”. They claim that if more people rode bicycles to work there would be fewer automobiles in the downtown section of the city and therefore less dirty air from car engines.

For several years this group has been trying to get city government to help bicycle riders. For example, they want the city to paint special lanes—for bicycles only—on some of the main streets, because when bicycle riders must use the same lanes as cars, there may be accidents. “Bike for a Better City” feels that if there were special lanes, more people would use bikes.

    But no bicycle lanes have been painted yet. Not everyone thinks it is a good idea. Taxi drivers don't like the idea—they say it will slow traffic. Some store owners on the main streets don't like the idea—they say that if there is less traffic, they will have less business. And most people live too far from downtown to travel by bike.

The city government has not yet decided what to do. It wants to keep everyone happy. On weekends, Central Park—the largest open place in New York—is closed to cars, and the roads may be used by bicycles only. But “Bike for a Better City” says that this is not enough and keeps fighting to get bicycle lanes downtown. Until that happens, the safest place to bicycle may be in the park.