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: Electric circuits provide energy for lightbulbs. Which of these prevents the flow of electrons?  
 (A) a circuit that is closed (B) a series circuit (C) a circuit that is open (D) a parallel circuit
Example Output: C

Example Input: A construction worker hit a solid steel wall with a sledgehammer, but the wall did not move. Part of the energy exerted by the worker  
 (A) was destroyed by the steel wall. (B) was converted into heat. (C) disappeared into the atmosphere. (D) was stopped by the hard surface.
Example Output: B

Example Input: The chemical symbol for niobium is Nb. Based on its symbol, what can be concluded about niobium?  
 (A) Niobium is a compound. (B) Niobium is an element. (C) Niobium is a metal. (D) Niobium is a mixture.
Example Output:
B