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 illness is always caused by an infection?  
 (A) a heart attack (B) a broken bone (C) influenza (D) cancer
Example Output: C

Example Input: When nitrogen-rich runoff from farms enters a nearby farm, it causes abundant growth of algal mat on the pond surface. As a result, underwater plants in the pond begin to die. Identify the limiting factor that is most responsible for causing the underwater plants to die.  
 (A) oxygen (B) sunlight (C) nitrogen (D) carbon dioxide
Example Output: B

Example Input: Which characteristic of sound makes it possible for bats to fly in the dark without hitting any objects?  
 (A) refraction (B) reflection (C) conduction (D) amplification
Example Output:
B