实验室利用过氧化氢、氯酸钾、高锰酸钾都可以制取氧气，其原因是（ ）
A．都属于氧化物
B．都含有氧气
C．都含有氧分子
D．都含有氧元素
答案：D
题目分析：根据物质的组成分析。
A、氧化物是由两种元素组成，并且其中一种元素为氧元素的化合物，而氯酸钾和高锰酸钾中虽都有氧元素，但它们都由三种元素组成，错误；
B、过氧化氢、氯酸钾、高锰酸钾三种物质都是纯净物，即由一种物质组成，故都不含氧气，错误；
C、氧气是由氧分子构成的，由于三种物质中都不含氧气，故也就不含氧分子，错误；
D、由于在化学变化中元素的种类不变，氧气是由氧元素组成的，因此制取氧气的物质中必须含有氧元素，根据三物质的组成可知，它们都含氧元素，正确。故选D
点评：熟练掌握常见物质的组成，以及物质的分类方法等知识，是解答本题的关键。
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