Oxidising and Reducing agents

The compound that can act both as oxidising and reducing agent is:

The compound that cannot act both as oxidising and reducing agent is

The compound that can work both as oxidising and reducing agent is

Which of the following can not act as both oxidising and reducing agent ?

The compound which could not act both as oxidising and reducing agent is

Account for the fact that SO_(2) can act as an oxidising and reducing agent while SO_(3) is only oxidising agent.