You Are Observing a Binary Star System and Obtain a Series

Multiple Choice

You are observing a binary star system and obtain a series of spectra of the light from the two stars. In this spectrum, most of the absorption lines shift back and forth as expected from the Doppler Effect. A few lines, however, do not shift at all, but remain at the same wavelength. How can we explain the behavior of the non-shifting lines?

A) there is a star in the system which is not moving at all: it is just sitting there
B) there is a planet orbiting the stars in the system
C) there are huge clouds of dust just behind this star system from our perspective
D) the lines come from interstellar matter between us and the star, not from the stars themselves
E) there is no explanation of this behavior: it is an unsolved mystery in science