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 of these will best separate a saltwater solution?  
 (A) using a paper filtering system (B) using chromatography (C) allowing the salt to settle (D) allowing the water to evaporate
Example Output: D

Example Input: When iron combines with oxygen in a moist environment, rust forms because of a  
 (A) chemical reaction. (B) physical reaction. (C) change in density. (D) change in temperature.
Example Output: A

Example Input: Natural selection is a mechanism that acts on individuals within a population. Which is a result of the process of natural selection?  
 (A) The reproductive rate of the population decreases. (B) Genetic similarity within the population increases. (C) Organisms are better adapted to their environment. (D) Adverse changes to the environment become more frequent.
Example Output:
C