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: Ella waves good-bye from the bus. What body system pulls on bones to let Ella wave her arm?  
 (A) muscular system (B) nervous system (C) digestive system (D) circulatory system
Example Output: A

Example Input: Martin is writing a report for science class about how the tilt of Earth affects living organisms. Which subject should Martin include in his report that is directly affected by Earth's tilt?  
 (A) night and day (B) cloud formation (C) phases of the Moon (D) changing of the seasons
Example Output: D

Example Input: A scientist is studying the effect of acid rain on oak trees. Of the numbers below, which would be the number of oak trees the scientist should use to obtain the most reliable data?  
 (A) 1 (B) 2 (C) 20 (D) 200
Example Output:
D