地理

To simplify things,chemists represent each element by a one- or two- letter symbols that you need to know to understand the material in this book .Just four elements-oxygen,carbon,hydrogen,and nitrogen-make up about 96% of your body weight and that of most other living things.
化學家以你需要代表每種要素知道理解在這本書裡的素材的一個一兩個字母符號。你的體重和其他大部分生物的組成元素有96%是僅僅由4個元素:氧，碳，氫和氮所構成。