What makes a pure substance different from a heterogeneous mixture?

a The components of a pure substance are mixed unevenly instead of evenly within the pure substance.
b Insoluble instead of soluble particles are suspended within the pure substance.
c Two liquids that normally are not mixable are mixed in a heterogeneous mixture.
d The components of a pure substance are single elements or compounds instead of a mixture of compounds.

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