All of the following are safety measures to prevent the delivery of hypoxic gas mixture to the patient except –
Location of oxygen valve after the N2O valve
Presence of a PIN index system to prevent wrong attachment of the N2O and oxygen cylinders
Location of a fail-safe valve downstream from the nitrous oxide supply source
All of the above
Pin index system is to prevent incorrect cylinder attachment (not to prevent delivery of hypoxic gas mixture).
Location of important parts :-


Flow control valve and flow meter (rotameter) of oxygen should be in downstream position (most distal).
Fail-valve is located downstream from N2O and other non-oxygen gas supply source, but not downstream from O2 supply source.