C-09-Q085mediumsingle_mcqFerrous oxide is represented by the formula—aFe2O3\mathrm{Fe_2O_3}Fe2O3bFe3O4\mathrm{Fe_3O_4}Fe3O4cFeO\mathrm{FeO}FeOdFeO2\mathrm{FeO_2}FeO2