Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef.

Fact 2: Coral is made of the exoskeletons of many thousands of tiny marine animals called polyps.

Given the two facts above, answer the question "Coral reefs are made up of the exoskeletons of thousands of what?" with the following options: 
- Absorb light
 - scarce resources
 - Chlorophyll
 - tiny polyps
 - to stay warm
 - animals
 - salt and water.
 - graptolites


 

    tiny polyps     