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

Fact 2: Corals, and invertebrates in general, are delicate creatures.

Given the two facts above, answer the question "What can secrete an exoskeleton that builds up to become a reef?" with the following options: 
- graptolites
 - some invertebrates
 - remains of prehistoric life
 - humans
 - insects
 - gastropod shells
 - barnacles
 - all invertebrates


 

  some invertebrates       