Which Is True About the Air at a High Altitude

Which is true about the air at a high altitude versus the air at a lower altitude?

A) Air contains a higher number of nitrogen molecules at higher altitudes.
B) There are fewer overall molecules of air at high altitude.
C) The percent of each gas present in air changes from high to low altitude.
D) The number of ozone molecules at high altitude is higher than at low altitude because the ozone layer is closer.