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: A student finds an animal in the wild. He decides it is a bird because it has  
 (A) fins. (B) scales. (C) feathers. (D) four legs.
Example Output: C

Example Input: The cheetah population around the world has decreased because of poaching and loss of habitat. The surviving population is found to have similar alleles. Which factor is endangering the species' survival the most?  
 (A) harmful genetic mutations (B) shrinking of the gene pool (C) decrease in competition (D) increase in adaptations
Example Output: B

Example Input: Bone cells need nutrients. Which two systems get nutrients to bone cells?  
 (A) the digestive system and the circulatory system (B) the circulatory system and the respiratory system (C) the respiratory system and the skeletal system (D) the skeletal system and the digestive system
Example Output:
A