Given a scientific question, generate a correct answer to it.

Input: Consider Input: The four major classes of biological macromolecules include carbohydrates, lipids, proteins, and what else?

Output: nucleic acids

Input: Consider Input: Matter can exist either as a pure substance or as a combination of what?

Output: different substances

Input: Consider Input: The common cold and flu are examples of human diseases caused by what tiny particles?
Output: viruses