Question

why fire need oxygen and why it cant do the work with co²

Answer 1

As carbon dioxide gas is non- combustible it cannot help in combustion, that is, burning of air. Oxygen is highly combustible and helps in the burning of fire which produces carbon dioxide.

Answer 2

as we know that every things that burn makes oxide and for the formation of oxide oxygen is required so oxygen is need by fire while co^2 makes a layer which disconnect the contact of fire from oxygen so oxygen is required for the burning process.