Even Though Cosmic Microwave Background Photons Outnumber Hydrogen Atoms by About

Multiple Choice

Even though cosmic microwave background photons outnumber hydrogen atoms by about 1 billion to 1 in the universe, the universe is still considered to be matter-dominated because the

A) photon energies are extremely small.
B) nature of the photons is such that they interact with nothing as they pass through the universe.
C) photons, while collectively carrying a large amount of energy, do not carry an equivalent amount of momentum and hence play little role in collisions with matter.
D) photons have no rest mass and hence can generate no gravity.