Instructions: 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").
Input: Fact1: Many corals secrete an exoskeleton that builds up to become a coral reef., Fact2: Coral is calcium carbonate with a trade of carotene., Question: A reef is made of (A) it keeps an organism warm (B) salt and water. (C) granite (D) magnesium (E) gastropod shells (F) ocean water (G) calcium carbonate (H) zinc
Output:
G