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    We do not know when people invented wheels. But once it was invented, knowledge of the wheelquickly across the world. It is easy to move things with wheels. The first example of wheels dated from about 3. 500 to 3. 000 BC. The cart or wagon was the first vehicle which had wheels. At that time, they were pulled by humans or animals. Today, there are still carts drawn by animals in many countries. These carts are mainly used to move things orpeople. Around 2, 000 BC, the chariot(双轮敝篷马车)appeared. It moved even faster than the cart, so it soon became an importantin the war and later was introduced to other ancient countries, such as Egypt, Persia, Greece and Rome.

    The invention of wheels is very important to theof new transportations. Without wheels, the invention of bicycles, motorcycles, cars, trains, and planes may not be possible.

    Traditionally, the President is the highest-paid public employee. A salary of $400. 000, along with other benefits, is paid to President annually. But President Obama's income for 2008 was far more than that, according to the White House.

    The White House recently published the tax returns(申报单)of Obama and Vice president Joe Biden. Tax returns are reports of tax that a person has to pay. They often include income information used to calculate the tax. In the US, people whose incomes are over a certain amount have to file tax returns every year. The tax returns of government officials are open to the public.

    This transparency(透明)helps to prevent corruption(腐败).

    From Obamas' tax returns, we can see that the President and first lady Michelle made about $2.66 million in 2008. Most of the money came from the sales of the President's two books Dreams From My Father and The Audacity of Hope. The two books, published in 1995 and 2006, are very popular and have been on the bestseller list for a long time.

    The Obamas' income is down from 2007, when they made $4. 2 million. However, that is still far more than former US president George Bush earned, who reported a total $ 719, 274 for his final year. The Obamas' income also greatly exceeded(超出)that of Biden, who reported a family income of $ 296, 256. That's about a tenth of what the Obamas earned. As Vice-president, Biden earns a salary of $200, 000, US families earn an average income of about $50, 000.

    The Obamas' tax returns also show that together, the President and his wife paid about $ 933. 000 in taxes. They gave $172, 050-nearly 6. 5 percent of their earnings-to different charity groups. In the US, if a person gives his money to churches and non-profit organizations, the law reduces his or her taxes. This is one reason why rich people in the US like to give money to charity.

    Some people from the country think that city life is the most interesting. They come to visit friends or go to a hotel in the city. They may stay for a few days or as long as a week.

    Some visitors think that foreign restaurants are the best part of city life. Others have the nicest time in museums. But many people think that movies or concerts are the most important attractions of the city. And most visitors like to go shopping for clothes or presents.

    In the city there are also many parks, different types of people, tall buildings, and other interesting things to see. Cities have the busiest streets and city life seems very busy, too.

    After a few days in the city, the visitors begin to think about the country. They remember the trees and flowers. Suddenly they do not want to be near the tallest buildings or the busiest streets. Now they want to return home in the country.