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: Planets in the solar system are in constant motion. What factor has the greatest effect on the orbits of the planets?  
 (A) the size of the planets (B) gravitational pull of the Sun (C) the composition of the planets (D) electromagnetic radiation from the Sun
Example Output: B

Example Input: A student was taking notes while the teacher was discussing cells. Which of these is a statement that the teacher most likely made?  
 (A) Most cells have the same shape. (B) The shape of a cell depends upon its function. (C) Cells that carry information are round and disk-shaped. (D) Blood cells that move through the body are long and thread-like.
Example Output: B

Example Input: Which of the following parts of the human body is most complex?  
 (A) heart (B) kidney (C) white blood cell (D) central nervous system
Example Output:
D