Excited hydrogen emits a spectrum of wavelengths in which the hydrogen-alpha line, at a wavelength of 656.28 nm, is a dominant feature. When a distant galaxy is observed from the Earth, the hydrogen-alpha line has a wavelength of 670.0 nm. If c is the speed of light, which of the following best describes the motion of the distant galaxy relative to the Earth?
a. The galaxy is moving towards the Earth with a speed of 2% of c .
b. The galaxy is moving towards the Earth with a speed of 23.7% of c .
c. The galaxy is moving away from the Earth with a speed of 2% of c .
d. The galaxy is moving away from the Earth with a speed of 23.7% of c .