It makes the ocean water saltier. It removes sand from the ocean floor. It causes ocean waves to become stronger. It creates a habitat for some ocean animals.
I gave my students this multiple choice question: Corals are small marine organisms that live in groups and make hard outer skeletons to protect their bodies. Over time, these outer skeletons can build up to make large coral reefs. Which of the following statements best describes one way the formation of a coral reef changes the ocean ecosystem?

Only one answer is correct among these 4 choices:
- It makes the ocean water saltier.
- It removes sand from the ocean floor.
- It causes ocean waves to become stronger.
- It creates a habitat for some ocean animals.

Could you tell me which one is correct?

It creates a habitat for some ocean animals.