Which statement best distinguishes a pure substance from a mixture?

Which statement best distinguishes a pure substance from a mixture?

• A. A pure substance contains a single type of particle; a mixture contains two or more substances.
• B. A pure substance has a variable composition, while a mixture has fixed composition.
• C. A pure substance is always a compound, while a mixture is always a solution.
• D. A pure substance cannot be separated into simpler substances by any means.

Answer: (A)