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]: Two atoms of oxygen combine to form pure oxygen gas. What is the electrical charge of the O_{2} molecules?  
 (A) positive (B) negative (C) neutral (D) variable
[EX A]: C

[EX Q]: The body is made up of many systems that interact with one another. For example, the skeletal system and the muscular system work together to help the body move. Which system works with the respiratory system to distribute oxygen to the body?  
 (A) muscular system (B) digestive system (C) lymphatic system (D) circulatory system
[EX A]: D

[EX Q]: Ducks live near ponds and lakes. The shape of a duck's foot helps it swim and walk on muddy ground. Which factor is most important in determining the shape of a baby duck's foot?  
 (A) the shape of the parent ducks' feet (B) the temperature of the pond water (C) the amount of mud in the bottom of the pond (D) the amount of rain that fell before the duck was born
[EX A]:
A