How long has the oxygen concentration of the atmosphere been at its current level?

developed and developing countriescountries are divided into two major categories by the united nations, which are developed countries and developing countries. the classification of countries is based on the economic status such as gdp, gnp, per capita income, industrialization, the standard of living, etc. developed countries refers to the soverign state, whose economy has highly progressed and possesses great technological infrastructure, as compared to other nations.