Seasonal Variations on a Planet's Surface Occur Because

Seasonal variations on a planet's surface occur because:

A) the planet's distance from the Sun varies periodically over the orbital path.
B) clouds alternately form and decay away in a periodic way.
C) the planet's axis of spin is tilted with respect to the perpendicular to the orbital plane.
D) volcanoes periodically cloud out the atmospheres of planets because of tidal interactions and distortions.