TASC Physical Science Practice Exam

The major components of the air at Earth's surface are nitrogen and oxygen, making up the vast majority of molecules, with small amounts of argon, carbon dioxide, and other trace gases. In dry air, nitrogen is about 78% and oxygen about 21% by volume (mole fraction). The remaining roughly 1% consists of argon, carbon dioxide, neon, helium, methane, and other trace gases. This is why the option listing about 78% nitrogen, 21% oxygen, and the rest as argon, CO2, and trace gases is the best description for the lowest atmospheric layer. The other options mismatch these proportions or misstate the presence of certain gases (for example, placing a large fraction of carbon monoxide or an unusually high amount of argon), which doesn’t reflect the typical composition of dry air at sea level.