__________is the oxidizing agent in the reaction below. \begin{aligned} & \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+6 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}+14 \mathrm{H}^{+} \rightarrow 2 \mathrm{Cr}^{3+}+\\ 3 \mathrm{~S}_{4} \mathrm{O}_{6}{ }^{2-}+& 7 \mathrm{H}_{2} \mathrm{O} \end{aligned} \text { A. }

S_{4} \mathrm{O}_{6}^{2-} \text { B. } \mathrm{Cr}^{3+} \text { C. H^{+ } } \text { D. } S_{2} O_{3}^{2-} \text { E. } \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}