下面是小宇同学学习《生物学》后，用下面如图表示相关概念间的包含关系，请你想一想，表中内容与如图相符的一组是（  ）

I have two friends. They are Bob and his sister Amy. Look! Bob

has a white T-shirt. He is not tall but fat. He is 10 years old.He

can ride a bike. Amy is the girl. She has a red skirt. She likes red

very much. She can fly a kite. She is 9 years old.

Oh! Look! The boy is me. I have a big mouth and long hair. I am

10 years old. I like swimming. I can swim very well. The little dog

is my friend, too. He can run and jump. He has big ears and a big

mouth. He is cute (可爱). I love him.

为了探究杠杆的平衡条件，物理老师带来了一个自制杠AB，从其上找一点C用支架支起来，如图所示．当小强依次在杠杆上的A₁、A₂、A等各点施加一个向下的力时，发现越来越容易提起水桶．根据这一实验现象大家提出的以下四个问题中，其中最有探究价值且易于探究的科学问题是（　　）