Pick from:
[i] graptolites;
[ii] some invertebrates;
[iii] remains of prehistoric life;
[iv] humans;
[v] insects;
[vi] gastropod shells;
[vii] barnacles;
[viii] all invertebrates;

Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef. Fact 2: Corals, and invertebrates in general, are delicate creatures. Given the two facts above, what can secrete an exoskeleton that builds up to become a reef?
Answer:
[ii]