You are presented with the question "What was formed by exoskeletons?" and the following answer choices: 
- heat produced
 - remains of prehistoric life
 - layers of fat
 - The area was once underwater
 - the Great Barrier Reef
 - the body's largest organ
 - sedimentary rocks
 - it keeps an organism warm

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and australia's coral reefs, including the Great Barrier Reef, are no exception, choose the best answer.


 

     the Great Barrier Reef    