下列物质的有关用途，由其化学性质决定的是    （   ）

    Mr. White lives in the countryside with his wife, Mrs. White. From Monday to Friday he works in a small office near his house. He is free on Saturdays and Sundays. He has a nice garden near his house. He likes growing flowers and he usually works in the garden on Saturdays and Sundays. The flowers are colourful and beautiful. Mrs. White likes them very much. She often helps Mr. White. They live happily together.

在探究杠杆平衡条件得实验中，我们曾多次改变力和力臂的大小，这样做的目的主要是为了（   ）