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.

已知一个样本中，50个数据分别落在5个组内，第一、二、三、四、五组数据的个数分别为2、8、15、20、5，则第四组的频率为（　　）

1895年，美国300家大垄断企业公司联合组成“全美制造商协会”。该协会设置有各种常务委员会，专门负责研究对内对外政策，并由顾问委员会同政府进行联系和沟通。这表明（    ）

如图所示，电源电压和电灯L的电阻均保持不变．当开关S闭合后，在变阻器滑片P从中点向下滑动的过程中，下列说法正确的是（　　）