$A$ violet compound of manganese $(A)$ decomposes on heating to liberate oxygen and compounds $(B)$ and $(C)$ of manganese are formed. Compound $(C)$ reacts with $KOH$ in the presence of potassium nitrate to give compound $(B)$. On heating compound $(C)$ with conc. $H_{2}SO_{4}$ and $NaCl$,chlorine gas is liberated and a compound $(D)$ of manganese along with other products is formed. Identify compounds $A$ to $D$ and also explain the reactions involved.