TASK DEFINITION: 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.
PROBLEM: 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

SOLUTION: C

PROBLEM: Many scientists are convinced that an increase of certain gases in the atmosphere is affecting global temperatures. What makes this hypothesis difficult to prove?
(A) Trees absorb these gases. (B) Global temperatures are rising. (C) Many variables affect global temperatures. (D) Many human activities produce these gases.

SOLUTION: C

PROBLEM: Planets in our solar system have different solar years. Which statement explains the cause of an Earth solar year?
(A) Earth rotates around the sun. (B) Earth revolves around the sun. (C) The sun rotates around Earth. (D) The sun revolves around Earth.

SOLUTION:
B