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 we now observe for the background radiation.
B) The cosmic microwave background radiation we observe is not cool-it is very hot, just like the Big Bang.
C) The Big Bang was a cold explosion, not a hot explosion-its temperature was the same as the current temperature we observe from the cosmic background radiation.
D) The cosmic background radiation we observe now is not from the Big Bang at all-it is from cold, intergalactic hydrogen clouds that cover the sky.