1.

About `"______"` by mass of the atmosphere consists of dioxygenA. `23%`B. `21%`C. `20%`D. `25%`

Answer» Correct Answer - 1
Oxygen exists in the free form as `O_(2)` molecules and make up `20.9%` by volume and `23%` by mass of the atmosphere .The proportion remains constant because the dioxygen consumed during respiration and combustion is continually replaced as a result of photosynthesis.


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