Anhydrous mercurous chloride can be prepared by

• A
  the reduction of $HgCl_2$ with $SnCl_2$ solution
• B
  the reaction of $HgCl_2$ with $Hg$
• C
  the reaction of $Hg$ with excess amount of $Cl_2$
• D
  the reaction of $Hg$ with concentrated $HCl$