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]: A drug company developed a pill to reduce cases of stomach virus. To test the medication, half of the participants got the pill with the new medicine and half got a sugar pill. Which term best identifies the participants who got a sugar pill?  
 (A) control (B) experimental (C) dependent variable (D) independent variable
[EX A]: A

[EX Q]: Similar types of cells form tissues. What is the next level of complexity formed from the grouping of tissues?  
 (A) organs (B) organelles (C) organisms (D) organ systems
[EX A]: A

[EX Q]: Three gases found in Earth's atmosphere are carbon dioxide, nitrogen, and oxygen. What is their order of abundance from greatest to least?  
 (A) nitrogen, oxygen, carbon dioxide (B) nitrogen, carbon dioxide, oxygen (C) oxygen, nitrogen, carbon dioxide (D) carbon dioxide, oxygen, nitrogen
[EX A]:
A