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: Which situation is the best example of an inherited trait?  
 (A) a boy playing with a ball (B) a bird building a nest (C) a horse pulling a wagon (D) a dog sitting on command
Example Output: B

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: Which of the following is the greatest source of wild fish for human populations?  
 (A) ponds (B) lakes (C) rivers (D) oceans
Example Output:
D