Which of these reactions is correct?

• A
  $Cl^{-} + Br_{2} \to Br^{-} + Cl_{2}$
• B
  Mohr's salt $\xrightarrow{NaOH \, \text{solution}} NH_{3} \uparrow (g)$
• C
  $K_{2}Cr_{2}O_{7} \text{ solution} \xrightarrow{SO_{3}} \text{Green colour solution}$
• D
  $FeCl_{2}$ $\xrightarrow{NaOH} (\text{ppt. coloured})$ $\xrightarrow{\text{Excess } NaOH} \text{Soluble complex}$