Question

part a
what is a pure substance?
a pure substance is composed of only one type of atom or one type of molecule
a pure substance is composed of two or more pure substances that have been mixed together in a variable proportion
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Explanation:

A pure substance has a definite and constant composition. It can be an element (composed of one - type of atom) or a compound (composed of one - type of molecule). A mixture of two or more pure substances in variable proportions is not a pure substance.

Answer:

A. A pure substance is composed of only one type of atom or one type of molecule