The substances became cooler and expanded. The substances physically changed the plastic. The substances became warm and stretched the bag. The substances chemically reacted and produced a gas.
A science teacher mixes two different substances in a plastic bag and seals the bag. The bag expands and breaks open. What is the most likely reason the bag expanded after the two substances were mixed?

Options:
- The substances became cooler and expanded.
- The substances physically changed the plastic.
- The substances became warm and stretched the bag.
- The substances chemically reacted and produced a gas.

The substances chemically reacted and produced a gas.