An aqueous solution of a salt (A) gives white crystalline precipitate (B) with NaCl solution. The filtrate gives a black ppt (C) when  $H_{2}S$  is passed into it ‘B’ dissolves in hot water and solution gives yellow ppt (D) on treatment with NaI and cooling. A does not give any gas with dil HCl but gives reddish brown gas on heating. The compounds B and C respectively are