题干

两个电荷量分别为q和-q的带电粒子分别以速度Va和Vb射入匀强磁场,两粒子的入射方向与磁场边界的夹角分别为30°和60°,磁场宽度为d,两粒子同时由A点出发,同时到达B点,如图所示,则(  )

A:a粒子带正电,b粒子带负电

B:两粒子的轨道半径之比Ra∶Rb=3∶1

C:两粒子的质量之比ma∶mb=1∶2

D:两粒子的速度之比Va∶Vb=1∶2

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C

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    No country in the world has more daily newspapers than the US. There are almost 2,000 of them, as compared with 180 in Japan, 164 in Argentina and 111 in Britain. The quality of some American papers is extremely high and their views are quoted all over the world. Famous dailies like the Washington Post or the New York Times have a powerful influence all over the country. However, they are not national newspapers in the sense that The Times is in Britain or Le Monde is in France, since each American city has its own daily newspaper. The best of these presents detailed description of national and international news, but many tend to limit themselves to state or city news.

    Like the press in most other countries, American newspapers range from the “sensational” which feature crime and gossip (闲谈), to the “serious”, which focus on factual news and the analysis of world events. But with few exceptions, American newspapers try to entertain as well as give information, for they have to compete with the attraction of television.

    Just as American newspapers satisfy all tastes, so do they also try to attract readers of all political parties. A few newspapers support extremist (极端主义的) groups on the far right and on the far left, but most daily newspapers try to attract middle-of-the-road Americans who are moderate (中立的). Many of these papers print columns by well-known journalists of different political and social views, in order to present a balanced picture.

    As in other countries, American newspapers can be either responsible or irresponsible, but it is generally accepted that the American press serves its country well and that it has more than once courageously exposed political scandals (丑闻) or crimes, for instance, the Watergate Affair (水门事件). The newspapers drew the attention of the public to the horrors of the Vietnam War.