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.

Ex Input:
The skin is the largest organ in the human body. Why is skin considered to be an organ?  
 (A) It is made of cells. (B) It acts as a barrier. (C) It is made of tissues. (D) It is part of an organism.

Ex Output:
C


Ex Input:
A container is filled with a mixture of sand and an unknown substance. What should a student do to determine whether iron particles are in the mixture?  
 (A) heat the mixture (B) filter the mixture (C) pour water into the mixture (D) pull a magnet through the mixture

Ex Output:
D


Ex Input:
Light bulbs vary in their ability to use electrical energy efficiently. Alicia compared the amount of electrical energy needed to light an incandescent bulb and a fluorescent bulb. Which units would Alicia use to compare electrical energy usage?  
 (A) newtons (B) joules (C) volts (D) watts

Ex Output:

B