Canadian Entry To Practice (CETP) Practice Test

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Approximately what percentage of air is oxygen?

  1. 10%

  2. 20%

  3. 30%

  4. 40%

The correct answer is: 20%

The percentage of oxygen in the Earth's atmosphere is approximately 20%. This proportion is significant for several reasons, including its role in supporting life through respiration for many organisms. The oxygen content is a stable component of the atmosphere and is essential for combustion processes, making it critical for various chemical reactions and biological functions. In the context of the other options, 10% would be insufficient for sustaining aerobic life, as most organisms rely on a higher concentration of oxygen for efficient energy production. Similarly, 30% and 40% are overestimates, as such high levels of oxygen could lead to toxicity in biological systems and also increase the risk of fire and combustion hazards. Thus, the accurate figure of around 20% reflects the balanced composition of the atmosphere necessary for life on Earth.

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