In Our Modern View of the Expansion of the Universe

Multiple Choice

In our modern view of the expansion of the universe, we understand that it is space that is stretching; individual galaxies don't speed away from each other as if they were rockets. In that case, why do galaxies show a red-shift?

A) as galaxies age, they get redder and redder
B) as space stretches, the waves of radiation in space also stretch and their wavelength increases
C) as space stretches, the presence of dark matter slows down the light between galaxies
D) as time goes on, waves in space encounter more and more dust, and get redder
E) astronomers cannot think of any good explanation for the red-shift if it's space that expands/stretches