Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

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