Which one of the following is an amphoteric oxide?

Provide the structures for:
$1.$ $MnO_4^{-}$
$2.$ $MnO_4^{2-}$

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:

Fill in the blanks:
$(a)$ The unit of magnetic moment is ......... .
$(b)$ $Mn^{2+}$ has .......... color.
$(c)$ $Fe_3H$ is a ............. type of compound.

Pick out the incorrect statement:

Among $Co^{3+}$,$Ti^{2+}$,$V^{2+}$ and $Cr^{2+}$ ions,one if used as a reagent cannot liberate $H_{2}$ from dilute mineral acid solution,its spin-only magnetic moment in gaseous state is $.....$ $B.M.$ (Nearest integer)