题目

The first chocolate was eaten by people in South America hundreds of years ago. In those days, the people did not really eat chocolate. They used beans to make a chocolate drink and they enjoyed it very much. Many years ago, the cocoa was brought to other countries and people came to love the taste of chocolate.In 1824, John Cadbury opened a small shop in Britain. One of the things he sold was chocolate drink. In 1831, he opened a factory to make chocolate drink. He wanted to encourage people to drink chocolate instead of other drinks. A few years later, a man called Joseph Fry found a way to 在图所示的电路中，电源电压U保持不变，定值电阻R=20Ω。闭合开关s，当滑动变阻器R’的滑片P在中点c时，电流表示数为0.4A，当移动滑片P至最右端时，电流表示数为0.3 A。则电源电压U与滑动变阻器R’的最大阻值为（ ） A．6V  10Ω      B．6V   20Ω     C．12V   20Ω     D．12V   40Ω