In the Present Universe, the "Mass Density" of Radiation, Using

Multiple Choice

In the present universe, the "mass density" of radiation, using Einstein's equation E = mc² for the photons, is

A) essentially zero since there is very little radiation left in the universe at the present time.
B) much greater than the mass density of matter leading to a radiation-dominated universe.
C) equal to the mass density of matter, since these parameters have remained balanced throughout the evolution of the universe as a consequence of the equipartition of energy.
D) much less than the mass density of matter, leading to a matter-dominated universe.