Why is there less oxygen concentration with an increase of altitude?

Question

please help!! :)

Answer 1

The same reason we don't fall off the earth...gravity!

Answer 2

Plants and trees consume CO2 and give out oxygen.So ,one of the main sources of oxygen are the trees and plants.They are found only on the surface.Even though the general composition of air(with its percentage of oxygen---21%) is almost the same(atleast upto 100 kms above),the concentration of O2 may be slightly more near the surface.That is why oxygen cylinders are required if  you climb a mountain.