Which of the following reactions is an example of autoreduction?

$1.<mspace\; width="1em"></mspace>{\mathrm{Fe}}_3{\mathrm{O}}_4+<mspace\; width="1em"></mspace>4\mathrm{CO}\to <mspace\; width="1em"></mspace>3\mathrm{Fe}+<mspace\; width="1em"></mspace>4{\mathrm{CO}}_2$

$2.<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>{\mathrm{Cu}}_2\mathrm{O}+<mspace\; width="1em"></mspace>\mathrm{C}\to 2\mathrm{Cu}+<mspace\; width="1em"></mspace>\mathrm{CO}$

$3.<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>{\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>\mathrm{Fe}\left(\mathrm{s}\right)<mspace\; width="1em"></mspace>\to \mathrm{Cu}\left(\mathrm{s}\right)<mspace\; width="1em"></mspace>+{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)$

$4.<mspace\; width="1em"></mspace>{\mathrm{Cu}}_2\mathrm{O}+\frac{1}{2}{\mathrm{Cu}}_2\mathrm{S}\to 3\mathrm{Cu}+\frac{1}{2}{\mathrm{SO}}_2$