The Cosmological Redshift of the Light from Very Distant Galaxies

The cosmological redshift of the light from very distant galaxies is caused by the

A) rotation of the universe around its center (faster at greater distances from Earth) .
B) expansion of space, stretching the photon's wavelength while the photon is traveling toward Earth.
C) absorption of blue light by interstellar dust between Earth and the galaxy so that only the red wavelengths reach Earth.
D) Doppler shift, in which the photon's wavelength is stretched by the galaxy's motion through space, away from Earth, while the photon is being emitted.