Which of the following reduction reactions are actually employed in commercial extraction of metals?

• A

  ${\mathrm{Fe}}_2{\mathrm{O}}_3+2\mathrm{Al}\to {\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{Fe}$

• B

  ${\mathrm{Cr}}_2{\mathrm{O}}_3+2\mathrm{Al}\to {\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{Cr}$

• C

  $2\mathrm{Na}\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]+\mathrm{Zn}\to {\mathrm{Na}}_2\left[\mathrm{Zn}(\mathrm{CN}{)}_4\right]+2\mathrm{Au}$

• D

  ${\mathrm{Cu}}_2\mathrm{S}+\mathrm{Pb}\to \mathrm{Cu}+\mathrm{PbS}\downarrow$