Fish have a two-chambered heart. Some fish are much larger than others. Fish need different amounts of oxygen. There are six types of fish in the aquarium.
I gave my students this multiple choice question: Several species of fish live in a classroom aquarium. Students notice that the gills of one type of fish move faster than the gills of the other fish. Which of these statements is the best hypothesis on this observation?

Only one answer is correct among these 4 choices:
- Fish have a two-chambered heart.
- Some fish are much larger than others.
- Fish need different amounts of oxygen.
- There are six types of fish in the aquarium.

Could you tell me which one is correct?

Fish need different amounts of oxygen.