Calomel is the name of

Which of the following mixtures forms chromic acid?

Correct statements from the following are:
$A$. Potassium dichromate is an oxidising agent and it oxidises $FeSO_4$ to $Fe_2(SO_4)_3$ in acidic medium.
$B$. Sodium dichromate can be used as primary standard in volumetric estimation.
$C$. $CrO_4^{2-}$ and $Cr_2O_7^{2-}$ are interconvertible in aqueous solution by varying the $pH$ of the solution.
$D$. $Cr-O-Cr$ bond angle in $Cr_2O_7^{2-}$ is $126^\circ$.
Choose the correct answer from the options given below:

The conversion of pig iron to steel frequently requires the addition of
$(I)$ oxygen or iron oxide
$(II)$ transition elements
$(III)$ inner transition elements
$(IV)$ silica

The total number of $Mn=O$ bonds in $Mn_2O_7$ is

The oxidation states of transition metal atoms in $K_{2}Cr_{2}O_{7}$,$KMnO_{4}$ and $K_{2}FeO_{4}$,respectively,are $x$,$y$ and $z$. The sum of $x$,$y$ and $z$ is