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: Forensic Medicine
The upper limit of safety for carbon monoxide in air is - 
Select and explain. 
A. 0.20%, B. 0.01%, C. 0.40%, D. 0.01%
B: 0.01%
Answer: B: 0.01% 
Explanation: The upper limit of safety of CO in air is 0.01%.