You are presented with the question "What can secrete an exoskeleton that builds up to become a reef?" and the following answer choices: 
- graptolites
 - some invertebrates
 - remains of prehistoric life
 - humans
 - insects
 - gastropod shells
 - barnacles
 - all invertebrates

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and corals, and invertebrates in general, are delicate creatures, choose the best answer.


 

  some invertebrates       