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 physical structure would best help a bear to survive a winter in New York State?  
 (A) big ears (B) black nose (C) thick fur (D) brown eyes
Example Output: C

Example Input: The shape of a riverbed changes over time as a result of which gradual process?  
 (A) erosion (B) dissolving (C) plate shifting (D) compaction
Example Output: A

Example Input: Which two body systems interact to produce antibodies that require essential amino acids?  
 (A) integumentary and circulatory systems (B) digestive and reproductive systems (C) endocrine and respiratory systems (D) digestive and immune systems
Example Output:
D