What is the purpose of inerting?

A. Is primarily to prevent flammable vapour/air mixtures in tanks and piping.
B. Is primarily to increase Oxygen content in vapour/air mixtures in tanks and piping.
C. Is primarily to increase Hydrocarbon content in vapour/air mixtures in tanks and piping.
D. Is primarily to increase Oxygen & Hydrocarbon content in vapour/air mixtures in tanks and piping.