You will be given a definition of a task first, then some input of the task.
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.

A student wants to use photography to study a process that occurs during one year. Which of these actions would be best for the student to study?  
 (A) a statue being weathered by acid rain (B) a glacier advancing and retreating (C) tree leaves changing colors (D) soil depth increasing
Output:
C