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: Anaesthesia
Gas cylinder with single pin index ? 
Select and explain. 
A. Oxygen, B. Air, C. Nitrogen, D. Entonox
D: Entonox
Answer: D: Entonox 
Explanation: Ans. is'd' i.e., Entonox