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.

[EX Q]: Calcium carbonate (CaCO_{3}) is considered a compound because it is made up of two or more  
 (A) molecules. (B) elements. (C) gases. (D) solids.
[EX A]: B

[EX Q]: The nucleus of MOST atoms consists of  
 (A) neutrons only (B) protons and neutrons (C) protons and electrons (D) neutrons and electrons
[EX A]: B

[EX Q]: One system of the human body works along with the skeletal system by assisting movement and maintaining posture. Which system is being described?  
 (A) respiratory (B) circulatory (C) muscular (D) digestive
[EX A]:
C