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
Pin index of oxygen is: March 2005 
Select and explain. 
A. 0, 5, B. 1, 5, C. 2, 5, D. 3, 5 Laughing
C: 2, 5
Answer: C: 2, 5 
Explanation: Ans. C: 2, 5 A gas-specific pin-index system is provided on small cylinders: pins on the yoke of the anaesthetic machine mate with holes drilled in specific positions on the valve of the cylinder to provide a mechanical means of preventing incorrect connection. Index positions: Oxygen 2 and 5 Nitrous oxide 3 and 5 Air 1 and 5.