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.

Example input: Which event can cause a large rock to become soil over time? 
 (A) decaying (B) an earthquake (C) a landslide (D) weathering
Example output: D
Example explanation: Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. Also, atmospheric factors change according to weather.
Q: 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.
A:
D