Use the equations for the area of a circle (a = πr²) and a sphere (sa = 4πr²) to explain why the insolation, or exposure to the sun's rays, is greater at the equator than at the poles.

a) Because the equator has a larger radius than the poles
b) Because the equator has a smaller radius than the poles
c) Because the equator has a larger surface area than the poles
d) Because the equator has a smaller surface area than the poles