在如图所示的电路中，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0.3A．则电源电压U与滑动变阻器R′的最大阻值为（　　）

备选人物：①诸葛亮    ②文天祥    ③袁隆平

示例：苏轼——宋代词人，虽屡遭挫折但依然豁达乐观。

阅读理解。

    Aunt Judy's birthday is coming. She will be 38 years old. Dad and Mum are going to take me to her house. She lives in Guangzhou. We are going to go there by train. Her birthday party will be on Saturday. We are leaving on Friday afternoon. And we are coming back on Sunday evening. I am going to give my dear Aunt Judy a picture . I am drawing it now. I am drawing some beautiful flowers. I am drawing a small dog, too. That's because she likes dogs. The picture will be very nice.