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
All of the following are used to maintain proper oxygen flow to the patient except – 
Select and explain. 
A. Placement of Oxygen flowmeter downstream of the Nitrogen flowmeter, B. A proportionater between N2 and O2 control valve, C. Different pin index for nitrogen and oxygen, D. Calibrated oxygen concentration analyses
C: Different pin index for nitrogen and oxygen
Answer: C: Different pin index for nitrogen and oxygen 
Explanation: Pin index safety system for cylinders is to prevent incorrect cylinder attachment (not to prevent hypoxia).