Use stream of consciousness to answer the following. By volume, dry air contains 78.09 % nitrogen, 20.95 % oxygen, 0.93 % argon, 0.04 % carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1 % at sea level, and 0.4 % over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

Answer this question based on the passage: what is the percentage of o2 in air?
To answer the above question, the relevant sentence is: By volume, dry air contains 78.09 % nitrogen, 20.95 % oxygen, 0.93 % argon, 0.04 % carbon dioxide, and small amounts of other gases. Final answer: 20.95 %.