Which has greater density? (densities of common substances"table.)

A-air set sea level

B - air at 20 km altitude

Your answer would be A, air at sea level.

Density is the amount of particles packed into a certain space. Something is more dense if there are more particles, something is less if there are less.

Air at sea level would be much more dense because of all of the air above it pushing it down, and packing more particles into a certain area. There is less above the air at 20km altitude, so it would get pushed down less and would therefore not be as compact.

This is also why, if you were to climb a mountain, there is less oxygen and the air "thins out".

Good luck!