The incorrect statement among the following is :-

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

Which alloy is used for making coins?

The transition element of $III-B$ group which forms an alum isomorphic with $Al_2(SO_4)_3 \cdot 18H_2O$ is $Sc$. The color of the compound $Sc_2(SO_4)_3 \cdot (NH_4)_2SO_4 \cdot 24H_2O$ is:

The presence of $Mn$ in steel produces

Which among the following statements is incorrect for interstitial compounds?
$1$. They are very hard and rigid
$2$. They have higher melting point than pure metal
$3$. They do not show conductivity
$4$. They are chemically inert