Solution:

The magnesium strip burns brightly in the air but continues to burn in the carbon dioxide environment.


This reaction is combustion and oxidation. What’s unusual, however, is that magnesium is reactive enough to be combusted and oxidized in a reaction with carbon dioxide:
2 Mg + CO2 -> 2 MgO + C

Under normal combustion/oxidation circumstances, oxygen is the reactant.
Carbon dioxide fire extinguishers work by smothering a fire in carbon dioxide. This is only an effective means of extinguishing a fire if carbon dioxide itself cannot be used as a fuel source.