The Center of Mass of a System of Particles Obeys

The center of mass of a system of particles obeys an equation similar to Newton's second law where:

A) is the total internal force and m is the total mass of the system
B) is the total internal force and m is the mass acting on the system
C) is the total external force and m is the total mass of the system
D) is the force of gravity and m is the mass of Earth
E) is the force of gravity and m is the total mass of the system