Answer the following question given this paragraph: 

Nitrogen is another common element found in living things. It is needed to form both proteins and nucleic acids such as DNA. Nitrogen gas makes up 78 percent of Earth’s atmosphere. In the nitrogen cycle, nitrogen flows back and forth between the atmosphere and living things. You can see how it happens in Figure below . Several different types of bacteria play major roles in the cycle.


Q: What gas comprises about three-fourths of earth's atmosphere?


A:
nitrogen