Why Does the Cosmic Microwave Background Appear to Be Slightly

Multiple Choice

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) Because this is the direction in which the Big Bang occurred,hence we are seeing the remnant of the explosion in this direction
B) Because the center of the Local Group of galaxies is in this direction and this is a bright source of microwaves
C) Because 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) Because the amount of dark matter in that direction in space has focused the radiation slightly by gravitational lensing,making this direction appear hotter