Question

which gas forms 21% of earths atmosphere?
a. argon
b. carbon dioxide
c. nitrogen
d. oxygen

Explanation:

To determine which gas forms 21% of Earth's atmosphere, we recall the composition of Earth's atmosphere. Nitrogen makes up about 78%, oxygen about 21%, argon about 0.93%, and carbon dioxide a very small fraction (around 0.04%). So the gas that forms 21% of the atmosphere is oxygen.

Answer:

D. oxygen