The orange colour of $K_2Cr_2O_7$ and purple colour of $KMnO_4$ is due to

$I$. $4FeCr_2O_4 + 16NaOH + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8H_2O$ (where $A = Na_2CrO_4$)
$II$. $2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$ (where $B = H_2SO_4$)
Compounds $(A)$ and $(B)$ are :

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:

An aqueous solution of $FeSO_4$,$Al_2(SO_4)_3$ and chrome alum is heated with excess of $Na_2O_2$ and filtered. The materials obtained are

Among $CrO$,$Cr_2O_3$,and $CrO_3$,the sum of spin-only magnetic moment values of basic and amphoteric oxides is . . . . . . . . $10^{-2} \ BM$ (nearest integer).
(Given atomic number of $Cr$ is $24$)

In which compound does chromium have a $+6$ oxidation state?