What makes a pure substance different from a heterogeneous mixture?

the components of a pure substance are mixed unevenly instead of evenly within the pure substance.
insoluble instead of soluble particles are suspended within the pure substance.
two liquids that normally are not mixable are mixed in a heterogeneous mixture.
the components of a pure substance are single elements or compounds instead of a mixture of compounds.

The components of a pure substance are single elements or compounds instead of a mixture of compounds  

Explanation:

This is the most nearly correct answer. The mixture could also consist of different elements or of elements and compounds.

A and B are wrong. The components of a pure substance are mixed evenly and do not have particles in suspension.

C is wrong. The statement is true, but it does not explain the difference between pure substances and mixtures.