阅读短文，回答问题。

Bear Amy,

   I want to be your friend. Let me introduce my family to you. My father is a doctor. He is tall and strong. My mother is a teacher. She is think and kind. My brother Sam is 6 years old. He's short and funny. We live in a big city. However, we usually go hiking on the weekends. I have a small and nice room. There is a bed and a desk in my room. I like reading books. I have many books on my desk. Welcome to my home.

                                                                                                                                                                        Yours,

                                                                                                                                                                          Lucy

质点是理想化模型．下列实例中，能把研究对象视为质点的是（   ）