Which of the following can be classified as a mixture?

a
pure air
b
pure water
c
pure nitrogen
d
pure gold

A pure substance is either an element (e.g. carbon, oxygen, nitrogen, sodium, lead etc.) or a compound (e.g. H2O, CO2, NH3, NaCl etc.). Air comprises elements such as N2, O2, argon and also compounds such as CO2, H2O(water vapour) etc. ... Hence, air is amixture and not a pure substance.

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.

i don't see a text, but i am gonna assume this *orbit* the curved path, usually elliptical, described by a planet, satellite, spaceship, etc., around a celestial body, as the sun.