Which one of the following pairs of substances when mixed,produces chlorine gas at room temperature?

• A
  $NaCl$ and $MnO_2$
• B
  $NaCl$ and $HNO_3$ (conc.)
• C
  $NaCl$ and $H_2SO_4$ (conc.)
• D
  $HCl$ (conc.) and $KMnO_4$