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.
--------
Question: 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.

Answer: C


Question: Compared to Earth, the Moon is  
 (A) drier. (B) larger. (C) heavier. (D) brighter.

Answer: A


Question: Elements are organized on the periodic table according to  
 (A) electronegativity values. (B) atomic mass. (C) atomic number. (D) the number of neutrons.

Answer:
C