Which of the following reactions leads to the formation of a substance in the colloidal state (in the presence of excess of $HCl$)?

• A
  $Cu + HgCl_2 \to CuCl_2 + Hg$
• B
  $2HNO_3 + 3H_2S \to 3S + 4H_2O + 2NO$
• C
  $2Mg + CO_2 \to 2MgO + C$
• D
  $Cu + CuCl_2 \to Cu_2Cl_2$