Which one of the following statements concerning electron energy bands in solids is true?
A. The bands occur as a direct consequence of the Fermi-Dirac occupancy probability function
B. Electrical conduction arises from the motion of electrons in completely filled bands
C. Within a given band, all electron energy levels are equal to each other
D. An insulator has a large energy separation between the highest filled band and the lowest empty band
E. Only insulators have energy bands
