What can iron oxide be used for?

Explanation:

oxide or ferric oxide is the inorganic compound with the formula Fe₂O₃. It is one of the three main oxides of iron, the other two being iron(II) oxide, which is rare; and iron(II,III) oxide, which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe₂O₃ is the main source of iron for the steel industry. Fe₂O₃ is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as Hydrous ferric oxide.

the answer is d.) potassium

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