Answer the following question given this paragraph: 

At the top of the troposphere is a thin layer of air called the tropopause. You can see it in Figure above . This layer acts as a barrier. It prevents cool air in the troposphere from mixing with warm air in the stratosphere.


Q: What thin layer of air acts as a barrier to prevent cool air from mixing with warm air in the stratosphere?

 Choices:

- methosphere

- endosphere

- liposphere

- tropopause

A:
tropopause
