State whether the following is oxidation or reduction.

${\mathrm{Fe}}^{3+} \to {\mathrm{Fe}}^{2+}$

State whether the following reaction is oxidation or reduction.

${\mathrm{O}}^{2-}\stackrel{ }{\to }\mathrm{O}$.

State whether the following reaction is oxidation or reduction.

${\mathrm{Cr}}^{7+} \to {\mathrm{Cr}}^{5+}$

State whether the following reaction is oxidation or reduction.

$\mathrm{Zn} \to {\mathrm{Zn}}^{2+}$