The orange-red color of an aqueous solution of $K_2Cr_2O_7$ changes to lemon yellow when treated with aqueous $KOH$. What is the reason for this?

The oxidising ability of the given anions follows the order:

The highest oxidation state of $Mn$ is exhibited in $Mn_2O_7$. The correct statements about $Mn_2O_7$ are:
$(A)$ $Mn$ is tetrahedrally surrounded by oxygen atoms.
$(B)$ $Mn$ is octahedrally surrounded by oxygen atoms.
$(C)$ Contains $Mn-O-Mn$ bridge.
$(D)$ Contains $Mn-Mn$ bond.
Choose the correct answer from the options given below:

Highest fluoride and highest oxide of $Mn$ are respectively

Copper sulphate is not used:

The oxoanion in which the oxidation state of the central atom is same as its group number in the periodic table is