题目内容
A、B两国都是WTO成员国，A国制定了对汽车尾气排放进行限制的技术法规，要求所有汽车必须安装催化转换器。在B国，同样的目的是通过使用柴油发动机实现。A国向B国出口的汽车，只要符合A国的相应技术法规即可，不必按B国相应法规对汽车进行调整，反之亦然。这体现了贸易技术壁垒(TBT)协议的基本原则中的()原则。
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已知一物块重P=100N，用F=500N的力压在一铅直表面上，如图所示，其摩擦系数fs=0.3，此时物块所受的摩擦力为()。
W: Well. Allen, the mountain-lake cycle tour is a two-day bicycle tour from Ottawa to Kingston and back. It is not a race, but more like an endurance test for recreational cyclists. People do it for fitness and fun and try to better their time every year. M: How long does it take to cycle one-way, Marilyn？
W: About seven to nine hours for the average cyclist. Racers can do it in four hours. But as I said, the tour is not intended to be a race.
M: How many people will be taking part this year？
W: This is our biggest year ever. We have twelve hundred cyclists registered.
M: I'd hate to be on the road when the cyclists start out.
W: Don't worry, we've introduced staggered starts to avoid these problems. Our first group will leave Ottawa at seven o'clock, and other groups will follow every twenty minutes.
M: Thank you, Marilyn. Now down to the starting line of the university.
Where did this interview take place？
一质量未知的托盘挂在弹簧上，如图所示，当盘上放一质量为m11；当盘上放一质量为m22。则弹簧的刚性系数K为()。
听力原文： A passenger jet headed for New York City was forced to land at Halifax, Canada this morning. The plane's transponder was broadcasting a signal indicating it had been hijacked. The Virgin Atlantic flight from London's Heathrow Airport was midway across the Atlantic when air traffic control officials in the US started receiving its signal: a signal which carried a special code indicating them was a hijacking in progress. Canadian fighter jets operating under NORAD, the joint Canada-US command, were scrambled to intercept the jet. According to NORAD, the pilots of the passenger jet repeatedly radioed in to tell controllers they were not being hijacked. But US officials erred on the side of caution. After the plane landed, armed police boarded it to ensure there were no hijackers. It was determined that the plane's transponder had mal-functioned, and was mistakenly transmitting code "7500" for hijacking.
Why the passenger jet was forced to land at Halifax, Canada？
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