Which of the following can act as an oxidising agent as well as a reducing agent?

I. H₂O₂                          II. H₂S                          III. SO₂                  IV. HNO₂

I. H₂O₂ undergoes oxidation to O₂ and also undergoes reduction to give H₂O (-1 oxidation state to 0 and -2)

II. S^2- undergoes only oxidation either to S, SO, or SO₄^2- in which oxidation number changes from -2 to 0, +4, or +6 

III. SO₂ (oxidation number of S is +4) can undergo oxidation and reduction. S is having highest oxidation state of +6. The element in a molecule having its oxidation state in the middle (i.e., > minimum and ( maximum) can be used as oxidant and as reductant both 

IV. HNO₂, (oxidation number of N is +3). Highest oxidation state of N is +5. So it can undergo both oxidant and reductant