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.

[Q]: One substance found on Earth is more stable as an elemental gas than as part of a solid compound. As a result, this substance is very abundant in the atmosphere. Which substance exhibits these traits?  
 (A) argon (B) chlorine (C) nitrogen (D) oxygen
[A]: C


[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.
[A]: B


[Q]: The temperature in a classroom is 15°C (59°F). The teacher turns the thermostat to 20°C (68°F) and keeps the door closed. Which of the following statements best describes the change that occurs as the temperature in the room increases to 20°C?  
 (A) Cold air molecules collide to generate heat energy. (B) Cold energy in the room transforms into heat energy. (C) The room becomes warmer as heat energy is added to the room. (D) The room becomes warmer as cold energy is pushed out of the room.
[A]:
C