When $\mathrm{MnO}_2$ is fused with $\mathrm{KOH}$ in presence of $\mathrm{KNO}_3$ 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 KI to compound $(C)$ whereas an acidified solution of compound (B) oxidises KI to (D). Identify (A), (B), (C) and (D).