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: Surgery
All post-operative patients must be supplemented with
oxygen concentration of at least
: 
Select and explain. 
A. 50-60%, B. 40-45%, C. 30-35%, D. 20-25%
C: 30-35%
Answer: C: 30-35% 
Explanation: None