Which of the Following Factors Prevents Protons and Electrons from Recombining

Which of the following factors prevents protons and electrons from recombining in the early phases of the Universe's history?

A) Photons have too much energy.
B) Protons and electrons do not have enough energy.
C) Protons and electrons are too far apart to collide.
D) Protons and electrons have too high of a temperature for gravity to attract them.