- The earth 's atmosphere is made of 21 percent oxygen by volume
- The earth 's atmosphere is made of 78 percent nitrogen by volume
- So, nitrogen and oxygen make up 99% of the earth's atmosphere by volume

- Nitrogen and oxygen make up 99% of the earth's atmosphere by volume
- Most means over 50% of something
- Hence, nitrogen and oxygen make up most of the earth's atmosphere

Hence, nitrogen and oxygen make up most of the earth's atmosphere
[Question]What two gases make up most of the Earth's atmosphere?
[Context]- The earth 's atmosphere is made of 21 percent oxygen by volume 
- Most means over 50% of something 
- The earth 's atmosphere is made of 78 percent nitrogen by volume