NO FAKE ANSWERS AND NO LINKS. How much oxygen does the atmosphere contain?

21%

Explanation:

To remember this percentage, use the amount of land and water on earth, like this: 3 (land) x 7 (water) = 21

(note: oxygen does not change depending on land or water amount)

The air in Earth's atmosphere is made up of approximately 78 percent nitrogen and 21 percent oxygen.

Explanation:

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plz mark brainliest

the answer is b.) hydrogen bonds between bases.

a. to encourage apoptosis