What is an inert condition?

 What is an inert condition?

1) When the oxygen has been eliminated from a tank 
2) when the oxygen is 10% by volume in a tank
3) A condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8% or less by volume by the addition of inert gas

Ans ____3) A condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8% or less by volume by the addition of inert gas