Oxygen concentration in Earth's atmosphere is roughly?
21%
By volume, the dry composition of Earth's atmosphere near the surface is approximately:
Nitrogen (N₂) ≈ 78.09%
Oxygen (O₂) ≈ 20.95% (commonly rounded to 21%)
Argon (Ar) ≈ 0.93%
Carbon dioxide (CO₂) ≈ 0.04%
Other trace gases (neon, helium, methane, krypton, etc.) make up the rest.
Hence the closest value among the options is 21%.
Hence, the correct answer is 21%.
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