Many corals secrete an exoskeleton that builds up to become a coral reef, and corals, and invertebrates in general, are delicate creatures. Given these facts, what can secrete an exoskeleton that builds up to become a reef among the following options:
- graptolites
 - some invertebrates
 - remains of prehistoric life
 - humans
 - insects
 - gastropod shells
 - barnacles
 - all invertebrates


 

  some invertebrates       