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
The fraction of O2 in inspired air in mouth-to-mouth respiration is 
Select and explain. 
A. 0.16, B. 0.19, C. 0.21, D. 0.26
A: 0.16
Answer: A: 0.16 
Explanation: The concentration of O2 in the expired air is 16%. That air is is transferred by mouth to mouth respiration.