Oxygen has an oxidation state of $+2$ in which of the following compounds?

Match the Column-$I$ with Column-$II$ for the oxidation states of the central atoms.

Column-$I$	Column-$II$
$A$. $Cr_2O_7^{2-}$	$1$. $+3$
$B$. $MnO_4^{-}$	$2$. $+4$
$C$. $VO_3^{-}$	$3$. $+5$
$D$. $FeF_6^{3-}$	$4$. $+6$
	$5$. $+7$

Fill in the blanks:
$(1)$ During the reaction $CN^{-} \to CNO^{-}$,......... is added.
$(2)$ The maximum positive oxidation number for any chalcogen compound may be ......... .

One mole of $N_2H_4$ loses $10 \ mol$ of electrons to form a new compound $Y$. Assuming that all nitrogen atoms appear in the new compound,what is the oxidation state of $N$ in $Y$? (There is no change in the oxidation state of hydrogen)

In the reaction,$MnO_{4(aq)}^{-} + Br_{(aq)}^{-} \rightarrow MnO_{2(s)} + BrO_{3(aq)}^{-}$,the correct change in oxidation number of the species involved is

$A$ compound contains atoms of three elements $A$,$B$ and $C$. If the oxidation number of $A$ is $+2$,$B$ is $+5$ and that of $C$ is $-2$,the possible formula of the compound is: