The Farther Away a Galaxy Is,the More Its Light Is

The farther away a galaxy is,the more its light is redshifted,as seen by us on Earth.This redshift-distance law is caused by

A) energy losses.The universe does not really expand,but photons lose energy as they travel,hence their wavelength lengthens.Photons from more distant galaxies have traveled farther,are more "tired," and so are more redshifted.
B) the expansion of space itself,which stretches the wavelength of the photon.The longer the photon travels,the more space expands and the more the photon is redshifted during the time it is traveling.
C) the gravitational redshift.Photons leaving a more distant galaxy have traveled farther through the galaxy's gravitational field,so they have lost more energy and are more redshifted.
D) the Doppler shift of light leaving a moving object.More distant galaxies are moving faster through space,so their light is more strongly Doppler-shifted toward the red wavelengths.