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: 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.
Example Output: D

Example Input: Farmers in Wyoming were concerned because some of their chickens were being preyed upon by hawks that lived in areas around their ranches. The farmers grouped together and hunted the hawks until they were no longer in their area. Which would most likely happen next?  
 (A) The chicken population would go down. (B) Populations of mice and rats would increase. (C) Another bird of prey would replace the hawk. (D) The chickens would have a lower rate of disease.
Example Output: B

Example Input: A perpetual motion machine is a theoretical device which, once started, continues to operate without any additional input of energy. Which statement describes why a perpetual motion machine is impossible to design?  
 (A) Energy can be converted into mass. (B) Friction reduces the efficiency in a system. (C) The amount of energy in a system remains constant. (D) Potential energy can be converted into kinetic energy.
Example Output:
B