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 student tosses a ball into the air. Which force causes the ball to fall back to the ground?  
 (A) gravity (B) magnetism (C) mechanical (D) friction
Example Output: A

Example Input: What is the main function of red blood cells?  
 (A) To fight disease in the body (B) To carry oxygen to all parts of the body (C) To remove carbon monoxide from all parts of the body (D) To produce materials which cause the blood to clot.
Example Output: B

Example Input: What two organ systems are involved in the transfer of oxygen to the cells from outside the body?  
 (A) skeletal and digestive (B) skeletal and respiratory (C) circulatory and digestive (D) circulatory and respiratory
Example Output:
D