Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef. Fact 2: Coral is made of the exoskeletons of many thousands of tiny marine animals called polyps. Given the two facts above, coral reefs are made up of the exoskeletons of thousands of what?

Available choices:
[i] Absorb light.
[ii] scarce resources.
[iii] Chlorophyll.
[iv] tiny polyps.
[v] to stay warm.
[vi] animals.
[vii] salt and water..
[viii] graptolites.
Answer:
[iv]