Which gas is described as comprising about 21% of the Earth's atmosphere by volume in the provided content?

Which gas is described as comprising about 21% of the Earth's atmosphere by volume in the provided content?

• A. Helium
• B. Oxygen
• C. Nitrogen
• D. Nitrous oxide

Answer: (D)

The amount spoken of as about 21% by volume refers to oxygen. In dry air, nitrogen is the most abundant component (about 78%), while oxygen makes up roughly one-fifth of the mixture, i.e., around 21%. Other gases like nitrous oxide, helium, or argon exist only in trace amounts, far less than 21%. Since “by volume” means the proportion of molecules present, oxygen is the gas described as ~21% of the atmosphere.

The amount spoken of as about 21% by volume refers to oxygen. In dry air, nitrogen is the most abundant component (about 78%), while oxygen makes up roughly one-fifth of the mixture, i.e., around 21%. Other gases like nitrous oxide, helium, or argon exist only in trace amounts, far less than 21%. Since “by volume” means the proportion of molecules present, oxygen is the gas described as ~21% of the atmosphere.