The chemical formula of magnetite is .

$\mathrm{A}$: ${\mathrm{Fe}}_2{\mathrm{O}}_3$

$\mathrm{B}$: $\mathrm{FeO}$

$\mathrm{C}$: ${\mathrm{Fe}}_3{\mathrm{O}}_4$

Enter your correct answer as $\mathrm{A}, \mathrm{B},$ or $\mathrm{C}$.

Consider the electrochemical cells(I and II) shown in the following figures and select the correct statement about these cells 

Give one word or a phrase for the following statement:

• Electrode used as a cathode in electrorefining of impure copper.

During electrolytic production of aluminium, the carbon anodes are replaced from time to time because