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: Living things are classified as producers or consumers according to  
 (A) their speed of movement (B) the size of their communities (C) how they obtain food (D) how they reproduce
Example Output: C

Example Input: Many scientists are convinced that an increase of certain gases in the atmosphere is affecting global temperatures. What makes this hypothesis difficult to prove?  
 (A) Trees absorb these gases. (B) Global temperatures are rising. (C) Many variables affect global temperatures. (D) Many human activities produce these gases.
Example Output: C

Example Input: Which of the following statements best describes the process of natural selection in a population?  
 (A) All individuals develop traits to match changes to their environment. (B) All individuals learn to adapt to specific changes that occur slowly in their environment. (C) Some individuals respond to their environment by mutating to make themselves more likely to survive. (D) Some individuals have genetic differences that make them more likely to survive and reproduce in their environment.
Example Output:
D