Answer the following question given this paragraph: 

Sure we can! If you've ever flown on a jet airplane, you've been in the stratosphere. Airplanes fly in the stratosphere, because there is less friction. They get better gas mileage and so costs are lower. There is also less turbulence, so passengers are happier. Can you identify the bottom of the stratosphere in the photo? It's where the clouds begin. Remember that all weather is in the troposphere.


Q: Planes fly in what layer of the atmosphere because there is less friction and turbulence?

 Choices:

- mesosphere

- ozone layer

- crust

- stratosphere

A:
stratosphere
