Answer the following question given this paragraph: 

When two or more pure substances are combined together, a mixture is formed. Unlike pure substances, mixtures have a variable composition. Variable composition indicates that the relative proportions of the mixtures components may vary, and they can be separated by physical methods. There are two main types of mixtures.


Q: Unlike pure substances, mixtures have a what?

 Choices:

- efficient composition

- unique composition

- complex composition

- variable composition

A:
variable composition
