The Energy of Incident Solar Radiation Reaching the Surface of Earth

The energy of incident solar radiation reaching the surface of Earth declines toward the poles from its maximum in equatorial regions. Why is this?

A) The tilt of Earth increases the reflection of solar radiation.
B) A given amount of solar radiation is spread over a larger area at higher latitudes.
C) Solar radiation travels a longer path through Earth's atmosphere at higher latitudes.
D) Both a and b
E) Both b and c