Which of the following most accurately describes the current understanding of energy? *

A. The greater the mass of all the components of any system, the greater the amount of energy it can generate.
B. As the energy of a system increases, so does the molecular motion of all parts of the system.
C. Energy can only be exchanged between systems that are in direct physical contact with each other.
D. Energy can never be created or destroyed, it can only be transferred between systems.