<prompt> All of the following are safety measures to prevent the delivery of hypoxic gas mixture to the patient exceptLocation of oxygen valve after the N2O valvePresence of a PIN index system to prevent wrong attachment of the N2O and oxygen cylindersLocation of a fail-safe valve downstream from the nitrous oxide supply sourceAll of the above  <response>: 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. 