The following can be determined for a solid substance.

I. The average kinetic energy ${E_{{{\rm{K}}_{{\rm{ave}}}}}}$ of the molecules
II. The total kinetic energy ${E_{{{\rm{K}}_{{\rm{tot}}}}}}$ of the molecules
III. The total potential energy ${E_{{{\rm{P}}_{{\rm{tot}}}}}}$ of the molecules

Which is/are equal to the internal energy of this solid substance?

A. I only
B. I and III only
C. II only
D. II and III only

D