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: Dental
The minimal concentration of oxygen that should be used with Nitrous oxide analgesia is: 
Select and explain. 
A. 10%, B. 20%, C. 30%, D. 40%
B: 20%
Answer: B: 20% 
Explanation: Nitrous oxide is a weak anaesthetic and while some patients can be brought to an unconscious level by mixture of Nitrous oxide and Oxygen containing at least 20% Oxygen.