Which of the following reactions occurs during iodometric titration in the laboratory?

• A
  $Cr_2O_7^{2-} + 14H^+ + 6I^- \to 2Cr^{3+} + 3I_2 + 7H_2O$; $I_2 + 2S_2O_3^{2-} \to S_4O_6^{2-} + 2I^-$
• B
  $MnO_4^- + 8H^+ + 5I^- \to Mn^{2+} + 2.5I_2 + 4H_2O$; $I_2 + 2S_2O_3^{2-} \to S_4O_6^{2-} + 2I^-$
• C
  $MnO_4^{2-} + OH^- + I^- \to MnO_2 + I_2$; $I_2 + 2S_2O_3^{2-} \to S_4O_6^{2-} + 2I^-$
• D
  $Cr_2O_7^{2-} + OH^- + I^- \to 2Cr^{3+} + I_2$; $I_2 + 2S_2O_3^{2-} \to S_4O_6^{2-} + 2I^-$