If the Cosmic Microwave Background Radiation Is the Radiation Left

If the cosmic microwave background radiation is the radiation left over from the Big Bang, why is its temperature only 3 K?

A) The universe has expanded and cooled to the temperature now observed for the background radiation.
B) The cosmic microwave background is only the cool tail of the background radiation, but the higher energies are blocked by galaxies and intergalactic clouds.
C) The Big Bang was a cold explosion, not a hot explosion-its temperature was the same as the current temperature observed from the cosmic background radiation.
D) The cosmic background radiation observed now is not from the Big Bang at all-it is from cold, intergalactic hydrogen clouds that cover the sky.