Question

why is air pressure greater at the earths surface?
a pressure is actually lesser at the earths surface.
b gravity pulls more molecules toward the earths surface.
c only smaller molecules are found at the surface, which increases pressure.
d molecules are smaller and less abundant there.

Explanation:

Air pressure is due to the weight of air molecules. Gravity pulls more air - molecules towards the Earth's surface, increasing the density and thus the pressure. Option A is incorrect as air pressure is greater at the surface. Option C is wrong because there is no such selectivity of only smaller molecules at the surface. Option D is incorrect as molecules are more abundant at the surface.

Answer:

B. Gravity pulls more molecules toward the Earth's surface.