Which of the Following Explains Why a Person Hears a Scale

Which of the following explains why a person hears a scale that sounds like it increases in pitch continually in Shepard Tones?

A) Timbre differences create an illusion of higher pitch, but all the tones are the same frequency.
B) White noise is presented simultaneously with the scale to phase out the decreased frequencies.
C) It is an illusion created by simultaneously sweeping different pure tones, which are an octave apart.
D) Notes one octave apart go into each ear, and then are switched, making the pitch seem higher though the two notes do not change.