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]: Which of the following is the best example of potential energy?  
 (A) a fallen rock (B) a burning match (C) a liter of gasoline (D) a red-hot piece of iron
[EX A]: C

[EX Q]: Scientists wonder how the Egyptian pyramids were built. They think that the huge blocks of stone may have been put into place by pushing them up a sloping pathway. The pathway is which type of simple machine?  
 (A) lever (B) pulley (C) inclined plane (D) wheel and axle
[EX A]: C

[EX Q]: The bonding of oxygen to red blood cells is important to the body because it enables the body to  
 (A) absorb nutrients from food. (B) make energy from light. (C) maintain a constant temperature. (D) perform aerobic respiration in cells.
[EX A]:
D