Which of the following statements are correct?

a. ${\mathrm{Fe}}^{2+}$ ion can reduce ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ to ${\mathrm{SO}}_4^{2-}$.
b. $\mathrm{KCl}$ is oxidized by ${\mathrm{Na}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$.
c. $∆\mathrm{G}$ for ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{2+}$ is more negative than for ${\mathrm{Cr}}^{3+}/{\mathrm{Cr}}^{2+}$.
d. ${\mathrm{VCl}}_5$ and ${\mathrm{VBr}}_5$ are stable in the air.