Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

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.


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