What is the oxidation state of chlorine in chlorous acid?

The oxidation number of carbon in $H_2C_2O_4$ is:

What is the oxidation number of $Mn$ in $MnO_{4}^{2-}$ ion?

What is the oxidation state of $S$ in $SO_4^{2-}$?

The oxidation states of three carbon atoms in carbon suboxide $(C_3O_2)$ respectively are

When $SO_2$ is passed through an acidic solution of potassium dichromate,chromium sulphate is formed. What is the change in the oxidation state of chromium?