Instructions: 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.
Input: Which causes the GREATEST change in a grassy field over time?  
 (A) The time of day (B) Amount of yearly rainfall (C) Number of birds nesting (D) Seasonal migration of animals
Output:
B