When MnO₂ is fused with KOH in the presence of KNO₃ as an oxidizing agent, it gives a dark green compound (A). Compound (A) disproportionates in acidic solution to give purple compound (B). An alkaline solution of compound (B) oxidises K I to compound (C) whereas an acidified solution of compound (B) oxidises K I to (D). Identify (A), (B), (C), and (D).