$CoO \cdot Al_2O_3$ is called

$A$ 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.

When $XO_2$ is fused with an alkali metal hydroxide in the presence of an oxidizing agent such as $KNO_3$,a dark green product is formed which disproportionates in an acidic solution to afford a dark purple solution. $X$ is

$X + KOH + O_2 \to Y + H_2O$,
$Y + H_2SO_4 \to Z + K_2SO_4 + MnO_2 + H_2O$
Identify $X, Y$ and $Z$ in the above reactions.

$CrO_3$ dissolves in aqueous $NaOH$ to give:

Which transition metal compound is used in a dry battery cell?