${CH}_{4}+{I}_{2} \underset{\text { Reversible }}{\stackrel{{hv}}{\longrightarrow}} {CH}_{3}-{I}+{HI}$
$(I)$ $CrO_4^{2-}$
$(II)$ $MnO_4^-$
$(III)$ $Cr_2O_7^{2-}$ ?