The Doppler effect applies to light waves as well as sound waves. Astronomers use this to measure the motion of stars and galaxies. In 1929, Edwin Hubble discovered that light from distant galaxies was shifted away from the blue end of the spectrum and towards the red end of the spectrum. Red light has a longer wavelength (and a lower frequency) than blue light. What could Hubble conclude about these galaxies?

A. The distant galaxies are growing smaller.
B. The distant galaxies are growing larger.
C. The distant galaxies are moving towards the Earth.
D. The distant galaxies are moving away from the Earth.