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

Fact 2: Good examples are Bubble coral, Elegance coral, Frogspawn and other Euphyllia corals.

Given the two facts above, answer the question "What is secreted by organisms like frogspawn to become a reef?" with the following options: 
- exocrine glands
 - food and shelter
 - exoskeleton
 - bones
 - Energy
 - Chemical energy
 - plasma
 - Energy.


 

   exoskeleton      