This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Mouth-to-mouth respiration provides an oxygen concentration of 
Select and explain. 
A. 16%, B. 20%, C. 22%, D. 24%
A: 16%
Answer: A: 16% 
Explanation: The concentration of O2 in the expired air is 16%.