They have no roots to anchor them to the ocean floor. They can live only where there is light. The pressure is too great for them to survive below 100 meters. If the algae lived below 100 meters they would be eaten by animals.
I gave my students this multiple choice question: Which BEST explains why green marine algae are most often restricted to the top 100 meters of the ocean?

Only one answer is correct among these 4 choices:
- They have no roots to anchor them to the ocean floor.
- They can live only where there is light.
- The pressure is too great for them to survive below 100 meters.
- If the algae lived below 100 meters they would be eaten by animals.

Could you tell me which one is correct?

They can live only where there is light.