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]: The relationship between ticks and a dog can best be described as  
 (A) mutualism. (B) competition. (C) parasitism. (D) commensalism.
[EX A]: C

[EX Q]: What is the main function of red blood cells?  
 (A) To fight disease in the body (B) To carry oxygen to all parts of the body (C) To remove carbon monoxide from all parts of the body (D) To produce materials which cause the blood to clot.
[EX A]: B

[EX Q]: Gaseous sulfur dioxide is a compound that combines with water in the atmosphere to form acid rain. What is the primary source of sulfur dioxide?  
 (A) volcanic emissions (B) combustion of fossil fuels (C) destruction of tropical forests (D) mining and mineral extraction
[EX A]:
B