What Causes the Loud Boom That We Hear as Thunder

What causes the loud boom that we hear as thunder?

A) supersonic movement of the leading end of a lightning bolt
B) supersonic expansion of air heated by a lightning bolt
C) supersonic movement of air pushed aside by a lightning bolt
D) supersonic vaporization of water droplets as they flash into steam when heated by the lightning
E) sudden generation of the sound wave as the "+" and "-" charges from the lightning connect