As the Temperature in the Early Universe Decreased, Pair Production

Multiple Choice

As the temperature in the early universe decreased, pair production reactions were confined to particles of smaller and smaller mass while annihilation reactions were unaffected. Why this difference?

A) Particles and antiparticles contain a certain amount of mass-energy independent of their temperature whereas the energy of a photon depends on its temperature.
B) Photons contain a certain amount of mass-energy independent of their temperature whereas the energy of particles and antiparticles depend on its temperature.
C) When particles collide, the interaction raises their temperature, but this does not happen when gamma-ray photons interact.
D) When gamma-ray photons collide, the interaction raises their temperature, but this does not happen when particles interact.