Explanation:

- The air we breathe is composed of several gases.

- Oxygen makes up a significant portion of this mixture.

- Oxygen accounts for approximately 21% of the Earth's atmosphere.

- Option:4 - 21 is the correct answer.

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- Option:1 - 28 is incorrect; it overestimates the oxygen percentage.

- Option:2 - 37 is incorrect; this figure is much higher than the actual value.

- Option:3 - 47 is incorrect; it's more than twice the true percentage of oxygen.