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

• A

  NaCI and ${\mathrm{MnO}}_2$

• B

  NaCI and ${\mathrm{HNO}}_3$ (conc.)

• C

  NaCl and ${\mathrm{H}}_2{\mathrm{SO}}_4$ (conc.)

• D

  HCI (conc.) and ${\mathrm{KMnO}}_4$