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: The pull of gravity from Earth has the greatest effect on  
 (A) comets. (B) the Sun. (C) asteroids. (D) the Moon.
Example Output: D

Example Input: A student examines a new watch design. The watch has a small solar cell on its face. The main function of the solar cell is to  
 (A) reduce the cost of the watch. (B) provide an energy source for the watch. (C) make the watch more attractive. (D) stop the watch from speeding up.
Example Output: B

Example Input: Which generates waves that are capable of traveling through a vacuum?  
 (A) a light bulb (B) a speaker (C) earthquake activity (D) volcanic eruption
Example Output:
A