What is the oxidation number of Carbon in $K_{2}C_{2}O_{4}$?

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalize your results?
$(1)$ $K\underline{I_3}$
$(2)$ $H_2\underline{S_4}O_6$
$(3)$ $\underline{Fe_3}O_4$
$(4)$ $\underline{C}H_3\underline{C}H_2OH$
$(5)$ $\underline{C}H_3\underline{C}OOH$

In Chromyl chloride,the oxidation state of chromium is $+6$.

The oxidation number of $Ba$ in barium peroxide is

The correct set of oxidation numbers of $N$ in $NH_4NO_2$ is

In chromite ore,the oxidation number of iron and chromium are respectively