What did Edwin Hubble discover about galaxies that allowed him to determine the expansion rate of the universe?

A. The farther away a galaxy was, the less its light was shifted toward the red end of the spectrum.

B. The farther away a galaxy was, the more its light was shifted toward the blue end of the spectrum.

C. The farther away a galaxy was, the more its light was shifted toward the red end of the spectrum.

D. The light from a galaxy was shifted a uniform amount toward the red end of the spectrum, regardless of its distance from Earth.