The Cosmic Microwave Background of Photons Is Observed to Have

Multiple Choice

The cosmic microwave background of photons is observed to have a temperature of about 3 K. What is believed to be the present temperature of the cosmic neutrino-antineutrino background?

A) less than 3 K-the microwave background is warmer because it received extra energy from the annihilation reaction that produced it.
B) 3 K-the expansion of the universe is the same for all particles in it.
C) 4 K-the universe was hotter when it became transparent to neutrinos.
D) 1,000,000 K-the neutrinos do not interact with matter and therefore have not been redshifted.