根据短文内容，选择正确答案。

    I am a student in China now. My name is Kelly Smith. I live with my parents, two sisters and a brother in Beijing. My parents teach English in No. 6 Middle School. I study in the same school. I am happy here, because I like my new school and the classmates. They are very kind to me. They like to play with me because I look different from them. I have blue eyes and long blond hair. They often say I look like a doll. Also I speak English well. so lots of students like to tam with me to improve (提高) their English.

保持二氧化碳化学性质的粒子是（      ）