Answer: There are two reasons as to why there are fewer air molecules at higher altitudes. One reason is that there is less air pressure at higher altitudes meaning that there is less air holding the molecules down, and secondly because gravity is not as strong higher in the air as it is closer to the surface. Both of these reasons combined allows these molecules to spread out more than they would at the surface. You did not provide any answer choices to choose from so I hope this has been a satisfactory answer :)
