Given the task definition and input, reply with output. In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").

Fact1: Many corals secrete an exoskeleton that builds up to become a coral reef., Fact2: An atoll is the third type of reef., Question: How are atolls built? (A) important habitats (B) By coral exoskeletons (C) by fish (D) with machines (E) by a type of eel (F) It used to be covered by water (G) The area was once underwater (H) reusing materials
B