Which statement correctly describes why a compound is a pure substance?

A compound is not a pure substance because each of its molecules is made up of the atoms of two or more different elements.
A compound is a pure substance because its molecule cannot be broken down into simpler particles by physical means.
A compound is not a pure substance because it is not an element, and only elements are pure substances.
A compound is a pure substance because it consists of two different elements, which are pure substances.