Which oxide of manganese is amphoteric in nature?

The color of acidified $K_2Cr_2O_7$ is not changed by:

The addition of metals like $Cr$,$Mn$,$W$ and $Ni$ to ordinary steel makes it

Which of the following has conductivity and appearance like that of copper metal?

Calomel is the name of

The fusion of chromite ore with sodium carbonate in the presence of air leads to the formation of products $A$ and $B$ along with the evolution of $CO_2$. The sum of spin-only magnetic moment values of $A$ and $B$ is .......... $B.M.$ (Nearest integer)
(Given atomic number: $C: 6, Na: 11, O: 8, Fe: 26, Cr: 24$)