题目内容
两个半径a和b，（a＜b）的同心导体球壳间电压为V0，若b固定，要使半球为a的球面上场强最小，则a与b的比之应该是（）
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Why do people in all countries ride buses to school, to work, and to far places Buses can go where trains and airplanes cannot. They do not cost as much to travel on. Even though they may not move as fast as airplanes and trains, they sometimes take people between two places faster. This is be- cause they may travel a more direct way, or may leave more often, or at better times.Buses come in different sizes. A small bus can carry only eight or ten people. A large one might have seats for fifty to seventy people and have standing room for more.Thousands of yellow school buses carry millions of children to school every year. Most of these buses have seats placed quite close together to fit in as many children as possible. Some schools buy their own buses. Others pay to use buses that belong to bus companies.City and town buses carry people for short distances. The seats of these are close together, and there is standing room as well. Most city buses have two doors so that people can get on and off quickly.long-distance buses go from state to state or country to country. They give people the chance to travel across huge stretches of land. These buses have comfortable seats that can be moved to different positions. They have places to store suitcases. Many have washrooms. All these things are important on a long trip. There are different kinds of buses mentioned in this passage except（）
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