UPSC NDA I 2021 General Ability Solved Paper

Concept:The exosphere is the outermost layer of Earth's atmosphere, composed of the lightest gases that can escape into space.Explanation:The exosphere begins at around $400\ \mathrm{km}$ and extends up to $10,000\ \mathrm{km}$.It contains very low density gases.The lightest elements rise to this layer due to diffusion.These are primarily Hydrogen ($\mathrm{H}$) and Helium ($\mathrm{He}$).Other heavier gases like Neon, Oxygen, and Argon are present in lower layers.Thus, the exosphere is dominated by $\mathrm{He}$ and $\mathrm{H}$.Answer:Helium and Hydrogen (Option A).