You are given a science question (hard-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: Several horses grazed in a fenced area across from a home. On rainy days, soil would wash down a slope and run toward the home. After the horses were moved a few years later, the soil no longer washed down when it rained. What could account for this change?  
 (A) The grass grew and kept the soil intact. (B) The fence kept the soil contained. (C) The soil was completely gone. (D) The amount of rain decreased.
Example Output: A

Example Input: Carolina wrens and painted buntings are two bird species that live in overlapping territories. They have different requirements for food and nesting. Which term best describes the relationship between the two species?  
 (A) parasitism (B) symbiosis (C) coexistence (D) mutualism
Example Output: C

Example Input: Which pair of human body systems functions most closely together?  
 (A) nervous and excretory (B) digestive and muscular (C) skeletal and circulatory (D) respiratory and cardiovascular
Example Output:
D