Which of the following ores is used for the production of $K_2Cr_2O_7$?

$KMnO_4$ decomposes on heating at $513 \ K$ to form $O_2$ along with

The ionic charges on chromate ion and dichromate ion respectively are:

In the structure of the dichromate ion,there is a :

The yellow colour of chromates changes to orange on acidification due to the formation of:

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: