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 is a safety measure for gas cylinders. What is the PIN index for nitrous oxide? 
Select and explain. 
A. 6-Mar, B. 5-Feb, C. 5-Mar, D. 6-Jan
C: 5-Mar
Answer: C: 5-Mar 
Explanation: Pin Index safety system: To discourage incorrect cylinder attachments, a pin index safety system has been adopted by cylinder manufacturers. The relative positioning of the pins and holes is unique for each gas. Pin Index safety system for medical gas cylinders: Gas Index Pins Air 1 and 5 CO2 mix (CO2 <7%) 2 and 6 CO2 mix (CO2 >7%) 1 and 6 Cyclopropane 3 and 6 Ethylene 1 and 3 He-O2 (<80%) 2 and 4 He-O2 (>80%) 4 and 6 Nitrous oxide 3 and 5 Nitrogen 1 and 4 Oxygen 2 and 5 Ref: Morgan, Jr. G.E., Mikhail M.S., Murray M.J. (2006). Chapter 2. The Operating Room: Medical Gas Systems, Environmental Factors, & Electrical Safety. In G.E. Morgan, Jr., M.S. Mikhail, M.J. Murray (Eds), Clinical Anesthesiology, 4e.