Discussion
Which percentage of dry air by volume is oxygen?
Answer: B
Oxygen constitutes 20.95 per cent of dry air by volume, close enough to 21 per cent for practical purposes. Its concentration is maintained by photosynthesis against the losses to respiration, combustion and oxidation of crustal minerals. The other values quoted belong to nitrogen, argon and carbon dioxide respectively, and mixing them up is a common slip that a moment's attention to the ordering prevents. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.
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