As altitude increases, what happens to air pressure?

it fluctuates.
it increases.
it decreases.
it stays the same.

It decreases

Explanation:

The air pressure tends to be higher on the places with the lowest altitudes, and lower at the places with higher altitudes. Basically, the air pressure is the wight of the air, and since the air is denser and heavier at the lower altitudes, the air pressure is higher, while on the higher altitudes the air is less dense, thus the air pressure is lower. So in practice we can expect that the air pressure in a low valley will be higher than the air pressure at the top of higher mountain.