某校实验小组在探究滑动摩擦力的大小跟哪些因素有关的实验中，提出了下列假设：①滑动摩擦力的大小可能与接触面积的大小有关；②滑动摩擦力的大小可能与接触面的粗糙程度有关；③滑动摩擦力的大小可能与接触面之间的压力大小有关；④滑动摩擦力的大小可能与运动速度大小有关。然后该实验小组做了如图所示的实验，以验证提出的假设。则这个实验验证的假设是    (    )

根据材料内容，选择正确答案。

    Jim, a young farmer, was once put into prison (监狱).

    One day, he got a letter from his mother. "I'm so worried about our farm," she wrote. "It's time to plant potatoes. I can't dig(挖) all the fields by myself. "

    Jim read the letter and became sad. "What can I do?" he thought. Then he had a good idea. He wrote to his mother, "Don't dig the fields. There's a pot of money in the earth. Don't plant potatoes until I come home."

    Some days later, Jim got another letter from his mother. It said, "Two days ago, about ten men came to our farm and dug all our fields. I can't understand it. It looks as if they were looking for something. What shall I do?"

    Jim smiled when he read his mother's letter. He wrote a letter to his mother at once. It was very short. Guess what it would say?

如图，吸收大部分营养物质的器官是（  ）

中国历史上第一个统一的中央集权的封建国家是（    ）