Why Does the Cosmic Microwave Background Appear to Be Slightly

Why does the cosmic microwave background appear to be slightly warmer in one direction in the sky and slightly cooler in the opposite direction?

A) The warmer direction is the direction in which the Big Bang occurred; hence, we are seeing the remnant of the explosion in this direction.
B) The universe is younger in one direction and therefore warmer.
C) The radiation in one direction is Doppler-shifted to shorter wavelengths by Earth's motion in space and to longer wavelengths in the other direction.
D) Large amounts of matter in the warmer direction have focused the radiation slightly by gravitational lensing, making this direction appear hotter.