This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anaesthetic equipments, Subject name: Anaesthesia
Gas cylinder with single pin index 
Select and explain. 
A. Oxygen, B. Air, C. Titrogen, D. Entonox
D: Entonox
Answer: D: Entonox 
Explanation: Entonox has an pin index number of 7 . Air - 1 , 5 ; Oxygen- 2 , 5 ; N20 - 3 , 5 . CO2 - 1 ,6 ; Cyclopropane - 3 , 6 ; Heliox - 4 , 6 . PIN index number is developed to prevent incorect cylinder attachment .