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.

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation: