Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef. Fact 2: Good examples are Bubble coral, Elegance coral, Frogspawn and other Euphyllia corals. Given the two facts above, answer the question "What is secreted by organisms like frogspawn to become a reef?" with the following options: - exocrine glands - food and shelter - exoskeleton - bones - Energy - Chemical energy - plasma - Energy.
A:
exoskeleton