Concept:Oxygen is blown through molten pig iron to oxidize and remove unwanted impurities during steelmaking.
Explanation:In the steel industry, oxygen gas
O2 is passed through molten pig iron in the basic oxygen process.
This oxidises carbon to carbon monoxide gas:
C+21O2→CO.
Sulphur is oxidised to sulphur dioxide,
SO2, and phosphorus is oxidised to phosphorus pentoxide,
P4O10.
These products escape as gases or are trapped in the slag, separating them from the purified steel.
Chlorine, nitrogen, and hydrogen cannot effectively remove these impurities from pig iron.
Therefore, oxygen is the substance used for this purification.
Answer:A. oxygen