Consider the reaction: 2mg + o2 → 2mgo
mg is ( ) and so is the ( )
o2 is ( ) and so it the ( )

*choices are oxidize, reduced, oxidizing agent, reducing agent.

In this redox reaction, the charge of Mg changes from zero to +2 while oxygen charge changes from zero to -2. In this case, Mg is being oxidized which makes it the reducing agent while O is being reduced making O2 the oxidizing agent.