Instructions: You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Input: The planets in the solar system revolve around the Sun because the Sun is
(A) made of hydrogen and helium. (B) older than other objects in the solar system. (C) the source of most heat and light energy. (D) the strongest source of gravity in the solar system.
Output:
D