The yellow colour solution of $Na_2CrO_4$ changes to orange-red on passing $CO_2$ gas due to the formation of $:-$

Which one of the following species is stable in aqueous solution?

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:

Which of the following statements is incorrect?

Silver halides are used in photography because:

Fusion of $MnO_2$ with $KOH$ in the presence of air or an oxidizing agent like $KNO_3$ forms a colored compound. Identify the product and its color.