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.

Ex Input:
Which organ is the site of gas exchange between the body and the environment?  
 (A) spleen (B) liver (C) lungs (D) pancreas

Ex Output:
C


Ex Input:
Which substance do all living things require to survive?  
 (A) blood (B) water (C) carbon dioxide (D) organic soil

Ex Output:
B


Ex Input:
Which gas could cause rust to form on a metal can?  
 (A) Hydrogen (B) Oxygen (C) Nitrogen (D) Helium

Ex Output:

B