You are given a science question (easy-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: A model of the Moon would be different from a model of Earth in that the Moon should  
 (A) be smaller than Earth. (B) have a source of light inside. (C) have a smoother surface than Earth. (D) have fewer impact craters on the surface.
Example Output: A

Example Input: Some people carry a mutated allele of a particular gene that makes them resistant to HIV infection and AIDS. Which best describes this type of mutation?  
 (A) beneficial (B) asexual (C) harmful (D) unicellular
Example Output: A

Example Input: The phases of the Moon are caused by the  
 (A) tilt of the Earth. (B) tilt of the Moon. (C) revolution of the Earth around the Sun. (D) revolution of the Moon around the Earth.
Example Output:
D