按照化学成分，天然气可分为哪几类？
参考答案：
（1）烃类气；（2）含硫气；（3）二氧化碳类气；（4）氮类气，共四类。
For professional athletes, ________ to the Olympics means that they have a chance to enter the history books.
A．access
B．attachment
C．appeal
D．Approach
