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: What gas in the air do we need to breathe in order to live?  
 (A) nitrogen (B) oxygen (C) carbon dioxide (D) hydrogen (E) water vapor
Example Output: B

Example Input: Which of these tools is best for recording observations outdoors?  
 (A) a ruler (B) a graph (C) a notebook (D) a calculator
Example Output: C

Example Input: What makes up most of a human skeleton?  
 (A) muscle (B) bone (C) skin (D) blood
Example Output:
B