Many corals secrete an exoskeleton that builds up to become a coral reef, and coral reefs are made up of the accumulated exoskeletons of the coral polyp. Given these facts, what do coral polyps form among the following options:
- lagoon
 - graptolites
 - a cuticle
 - barnacles
 - bioaerosols
 - cave
 - coral reef
 - salt and water.


 

       coral reef  