When looking at the side view of a distant galaxy, we notice that spectra from the left half is red-shifted while the spectra from the right half is blue-shifted. This phenomenon indicates:
a) The galaxy is rotating rapidly.
b) The galaxy is stationary.
c) The left half of the galaxy is moving away, while the right half is moving towards us.
d) The left half of the galaxy is moving towards us, while the right half is moving away.