Answer the following question given this paragraph: 

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.


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

 Choices:

- membrane

- mucous

- scales

- exoskeleton

A:
exoskeleton
