What is the oxidation state of $Cr$ in potassium dichromate?

In which among the following compounds,the oxidation number of nitrogen is $ +5 $?

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results?
$(a)$ $K\underline {I_3}$
$(b)$ $H_2\underline {S_4}O_6$
$(c)$ $\underline {Fe_3}O_4$
$(d)$ $\underline {C}H_3\underline {C}H_2OH$
$(e)$ $\underline {C}H_3\underline {C}OOH$

What is the oxidation number of $S$ in $SO_3^{2-}$?

Suggest a list of substances where carbon can exhibit oxidation states from $-4$ to $+4$ and nitrogen can exhibit oxidation states from $-3$ to $+5$.

What is the oxidation state of $Fe$ in $Fe_3O_4$?