Q: Altitude is height above what?


Read this paragraph and choose the correct option from the provided answers:

Altitude is height above sea level. The density of air decreases with height. There are two reasons. At higher altitudes, there is less air pushing down from above. Also, gravity is weaker farther from Earth's center. So at higher altitudes, air molecules can spread out more. Air density decreases. You can see this in Figure below .

 Choices:

- the earth's core

- the earth's surface

- the water table

- sea level


A:
sea level