如图是汽车、摩托车油量表（实际上就是电流表）原理示意图，R′是定值电阻，R是滑动变阻器，则（　　）

   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.