This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anesthesia Machine, Subject name: Anaesthesia
46 yr old chronic smoker came to emergency dept with a history of difficulty in breathing & fever since 1day, postgraduate tells you to connect face mask to which pin index number 
Select and explain. 
A. 1, 5, B. 2, 5, C. 3, 5, D. 1, 6
B: 2, 5
Answer: B: 2, 5 
Explanation: - Machines are usually equipped with one or two E type cylinders that hang on specific hanger yokes. - The medical gas pin-index safety system ensures that the correct medical gas cylinder is hung in the correct yoke. Gas Index Pins Oxygen 2,5 Nitrous oxide 3,5 Cyclopropane 3,6 O2-CO2 (CO2<7.5%) 2,6 O2-CO2(CO2>7.5%) 1,6 O2-he(He>80.5%) 4,6 O2-he(He<80.5%) 2,4 Air 1,5 Nitrogen 1,4 N2O-O 7