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
In low concentration of carbon monoxide (40/) the commonest symptom is : 
Select and explain. 
A. Nausea, B. Headache, C. Muscular weakness, D. None
D: None
Answer: D: None 
Explanation: D i.e. None