instruction:
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:
The transfer of heat in the atmosphere by winds that circulate between the equator and the poles is an example of  
 (A) radiation. (B) insulation. (C) convection. (D) conduction.
answer:
C


question:
Which property of an element is determined by adding the number of protons and number of neutrons?  
 (A) atomic mass (B) atomic radius (C) electrical charge (D) electrical potential
answer:

A