This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 2, Subject name: Physiology
The PO2 (in mm Hg) inside skeletal muscle cells during exercise is closest to: 
Select and explain. 
A. 3, B. 10, C. 20, D. 30
A: 3
Answer: A: 3 
Explanation: The PO2 (in mm Hg) inside skeletal muscle cells during exercise is closest to - 3mm Hg The PO2 decreases severly due to muscle's oxygen demand is very high.