Incoming Solar Radiation in Middle Latitudes Is Less in Winter

Incoming solar radiation in middle latitudes is less in winter than in summer because

A) the sun's rays slant more and spread their energy over a larger area.
B) the sun's rays are weakened by passing through a greater thickness of atmosphere.
C) the cold dense air lowers the intensity of the sun's rays.
D) all of the above
E) a and b