Why does the Earth spin faster at the equator than the poles?

a
Due to the temperature differences on Earth, the land/sea near the equator goes at a faster rate. The Earth spins faster there only during the summer months.
b
The Earth is much wider at the equator than at the poles. It must cover a larger distance in the same amount of time (24 hours).
c
Due to the tilt of the Earth on its axis, the equator gets more sunlight -- therefore making it spin faster.