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现有C、H、O、Ca、S五种元素，请选用其中一种或几种元素写出符合下列

要求的化学式各一个：

(1) 我国实施的“西气东输”工程输送的“气”是 ；

（2）实验室制取氧气的一种氧化物药品是 ；

（3）常用于金属除锈的酸是 ；

(4) 常用于食品干燥剂的是 ；