I      to Rongjiang last year．But I      never     to Chongjiang．  

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    Chen Jie and Chen Lin are sisters. Chen Jie is 14. She's 2 years younger than Chen Lin. But she's taller than Chen Lin. Chen Jie is 46 kilograms. She's l kilogram heavier than Chen Lin. Chen Jie likes sports. She plays sports every day. So she's stronger than Chen Lin. Chen Jie has short hair, but Chen Lin has long hair. They often wear the same size shoes. They wear size 36. They love each other.

既能表示一种元素，又能表示该元素的一个原子，还能用于表示该物质化学式的是（    ）