Aqueous solution of (M) +NH42 S→ yellow ppt (B) . The salt M can be

(a) CdS Yellow ↓+NH42 S⟶CdS Yellow ↓NH42 S ⟶CdS↓ Insoluble  (b) SnS2↓+NH42 S⟶SnS2 Yellow ↓NH42 S ⟶NH42SnS3 Soluble  (c) Cd2++NH42 S⟶CdS Yellow ↓CdS↓ Insoluble (d)Sn2++(NH4)2S→SnS↓→(NH4)2S(NH4)2SnS3                             Brown               (Ammonium stannate)                                                                (soluble) CdS is yellow and SnS is Brown or Black.