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

Fact 2: Australia's coral reefs, including the Great Barrier Reef, are no exception.

Given the two facts above, answer the question "What was formed by exoskeletons?" with the following options: 
- 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


 

     the Great Barrier Reef    