$A$ group of acidic oxides is

Alkaline oxidative fusion of $MnO_2$ gives $A$ which on electrolytic oxidation in alkaline solution produces $B$. $A$ and $B$ respectively are:

Superconductors are derived from compounds of

When a chromite ore $(A)$ is fused with sodium carbonate in free excess of air and the product is dissolved in water,a yellow solution of compound $(B)$ is obtained. After treatment of this yellow solution with sulphuric acid,compound $(C)$ can be crystallised from the solution. When compound $(C)$ is treated with $KCl$,orange crystals of compound $(D)$ crystallise out. Identify $A$ to $D$ and also explain the reactions.

The solution of $MnO_4^-$ is purple-coloured due to:

Which oxide of transition metals exhibits electrical conductivity as high as that of metals?