#### Answer

(b) all add to zero.

#### Work Step by Step

The correct answer is: (b) all add to zero.
According to Newton's third law, every force has an equal and opposite force. Suppose that object A exerts a force on object B and let's assume that this force is an internal force in the system.
The internal force exerted on object A by object B is the interaction partner of the force exerted on object B by object A. These two forces are equal in magnitude and opposite in direction. Therefore, the sum of these two forces add to zero.
Since every internal force in a system has an interaction partner that is also an internal force in the system, the sum of the internal forces all add to zero.