When a brown compound of manganese $(A)$ is treated with $HCl$,it gives a gas $(B)$. The gas,when taken in excess,reacts with $NH_{3}$ to give an explosive compound $(C)$. Identify compounds $A$,$B$,and $C$.

(A) $A: MnO_{2}, B: Cl_{2}, C: NCl_{3}$ (explosive).
The reaction of manganese dioxide with hydrochloric acid is:
$MnO_{2} + 4HCl \rightarrow MnCl_{2} + Cl_{2} + 2H_{2}O$
The reaction of excess chlorine with ammonia is:
$NH_{3} + 3Cl_{2} \rightarrow NCl_{3} + 3HCl$
Thus,the compounds are $MnO_{2}$,$Cl_{2}$,and $NCl_{3}$.