阅读理解。
Moving out, Moving back
In the countries all over the world, people are leaving the countryside in large numbers to find jobs in
the cities. They leave because the countryside is much poorer than the city, and often there isn't much
work there. Services such as hospital and transport are usually much better in cities than in the countryside. People hope that their lives will improve when they move to the city.
But in the big cities of Europe and America-London, Pairs, New York, people are moving out of the
city. These are usually families who have some money and want to live a quieter life. They are tired of the
noise and the dirt of the city, tired of the crowded streets, trains and buses. They don't want to sit in traffic any more. They want a house with a garden, and clean air that isn't polluted. So they move out. Some don't go very far, just a little way out of the city, to the town near the cities, and other people move to the
real countryside with sheep, cows and green fields!There,they start new lives and try to make new friends.
Not all those who move from the city to the countryside are happy. After two or three years, many
people who have done this feel that it was a big mistake. They don't make so much money and there isn't much to do. Country people are very different and aren't always very friendly.
As a result, quite a lot of people who have moved to the countryside move back to the city. " It's
wonderful to see crowds in the streets and cinema lights." they say, "I'm so glad I'm back in the city!"
1. Is there often much work to do in the countryside?
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2. What kind of families are moving out of the city?
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3. Why are the people moving to the countryside?
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4. Do the people feel that they make a mistake when they move to the countryside?
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5. What can we learn from the sentence "It's wonderful to see crowds in the streets and cinema lights"?
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，运行速率为
，向心加速度为
，轨道所在处的重力加速度为
。地球的半径为R，第一宇宙速度为
，地球赤道上的物体随地球自转的向心加速度为
，地球表面的重力加速度为g。则下列结果不正确的是（ ） 
