Q: What do corals secrete that builds up to become a coral reef?


Read this paragraph and choose the correct option from the provided answers:

Cnidarians include jellyfish and corals. They are aquatic invertebrates. They have tissues and radial symmetry. They also have tentacles with stingers. There are two cnidarian body plans: the polyp and the medusa. They differ in several ways. Many corals secrete an exoskeleton that builds up to become a coral reef.

 Choices:

- membrane

- mucous

- scales

- exoskeleton


A:
exoskeleton