The cosmic background radiation is A)
the flux of visible radiation in empty space, contributed by all visible stars in the universe.
B)
the radio noise generated by Earth-bound transmitters, spreading out into space since about 1920.
C)
the beam of atomic nuclei known as cosmic rays that continuously rain down upon the Earth from all directions in space.
D)
low-intensity radio noise, with a 3 K blackbody temperature, almost uniform in intensity in all directions.