如图所示，在火星与木星轨道之间有一小行星带．假设该带中的小行星只受到太阳的引力，并绕太阳做匀速圆周运动．下列说法正确的是（   ）

写出下列句子的意思。

   It began with one red paper clip(回形针)and ended with a house. Kyle MacDonald, a 26-year-old Canadian got a house with three bedrooms through 14 trades(交易)that started with a single red paper clip.
   Now more and more young people around the world like this kind of trading activities. They set up websites and trade on the Internet. People trade things they do not usually use with other people. A website called Peerflix allows people to trade their old possessions. These traders usually prefer face-to-face trading, in this way they don't have to worry about who is going to pay for the delivery(递送).
   Last year, a young girl in Beijing decided to follow the example of Kyle. She started with a paper clip and hoped to get a house in the end．After several trades she now has a piano which is worth more than 10, 000 yuan. But she may still have a long way to go.
   Maybe you can't understand why people are doing this. As Kyle said, "What's more important to a man in the desert—one million dollars or a glass of water?" Kyle's words probably show why these trades are popular among young people.

根据定积分的定义，${\int }_0^2{x}^{2}dx$=(     )