The right answer is nitrogen.
The chemical composition of the atmosphere essentially comprises nitrogen (78%), oxygen (21%), rare gases (Argon, Neon, Helium ...) and in the lower layers, water vapor and carbon dioxide.
The constituents of atmospheric air can be classified into two categories:
* constituents such as nitrogen, rare gases, whose concentration is constant, at least in the lower layers of the atmosphere.
* Constituents whose content varies in the atmosphere, such as carbon dioxide and especially water vapor.
