Why Is Either Pure Oxygen or Oxygen Diluted with Argon

Why is either pure oxygen or oxygen diluted with argon used in a converter instead of air?

A) The oxygen concentration is too low to function efficiently at removing impurities.
B) The carbon monoxide in air reacts with the iron to form a volatile, and toxic, iron carbonyl.
C) The nitrogen in air will react with iron to form iron nitride that will make the iron brittle.
D) Because it's cheaper.
E) The carbon dioxide in air will cause the iron to oxidize and form rust.