Why Is the Cosmic Microwave Background (CMB) So Cold If

Why is the cosmic microwave background (CMB) so cold if the early universe was so hot?

A) Enough time has passed for matter in the universe to release enough heat to cool down.
B) The CMB constantly interacts with atoms, which effectively cool down the photons.
C) The expansion of the universe has redshifted those photons to an effectively cooler temperature.
D) Misleading; The Big Bang was initially hot but the CMB was released from cold material much later on.