What causes cosmological redshift of photons that reach us from distant galaxies?
A) The photons have moved from high gravitational field regions toward lower fields,thereby becoming reddened.
B) The photons were emitted from the galaxies much earlier in time when the overall temperature of matter was much lower.Hence,the observed photons are redder,the farther away from Earth that they were produced.
C) The photons have traveled across space that has been expanding and their wavelengths have expanded with it,becoming redder.
D) The photons were emitted by objects that were moving rapidly away from us,and thereby have been reddened by the Doppler effect.
