CO (carbon monoxide) is a poisonous gas that can be broken down by chemical means. It is a colorless, odorless, and tasteless gas that is produced by the incomplete combustion of fuels. CO can be broken down by a variety of chemical reactions, including:

* Reaction with oxygen: CO + O2 → CO2

* Reaction with water: CO + H2O → CO2 + H2

* Reaction with metal oxides: CO + metal oxide → metal + CO2

Cu, Ce, and Ca are all metals. They cannot be broken down by chemical means.