What is meant by "reverberation" in an auditorium?
(a) A distinct repetition of sound heard after at least 0.1s from a distant wall
(b) Gradual decrease in loudness of sound in a hall due to absorption by walls and air
(c) Persistence of sound in an auditorium caused by multiple reflections even after the source has stopped
(d) Bending of sound waves around the edges of obstacles inside the hall