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

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

Ex Output:
nucleic acids

Ex Input:
How is a reaction described when the given conditions favor formation of products?

Ex Output:
spontaneous

Ex Input:
What kind of structure contains the largest known single cell?

Ex Output:
egg