The Cosmic Background Radiation Is

Multiple Choice

The cosmic background radiation is

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