The Apparent Reddening of Light from Stars After Its Passage

The apparent reddening of light from stars after its passage through the interstellar medium is caused by

A) the additional contribution to this starlight by emission from hydrogen gas in the interstellar medium.
B) preferential scattering of blue starlight by dust grains.
C) Zeeman shift of the light by the powerful magnetic fields existing within the interstellar medium.
D) scattering of this light from rapidly moving material; the light is Doppler-shifted toward the red end of the spectrum.