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.

Example Input: Some bacteria use iron found in their systems as a means to guide their movement. Which stimulus most likely triggers this movement mechanism?  
 (A) magnetism (B) oxygen (C) light (D) heat
Example Output: A

Example Input: Thomas Edison most likely invented the light bulb by using  
 (A) lit candles. (B) atomic models. (C) reflected light. (D) the scientific method.
Example Output: D

Example Input: Which is electrically attracted to positively-charged particles in an atom?  
 (A) neutrons (B) electrons (C) protons (D) nucleus
Example Output:
B