If the total momentum of a system is changing:

(a) particles of the system must be exerting forces on each other

(b) the system must be under the influence of gravity

(c) the center of mass must have a constant velocity

(d) a net external force must be acting on the system

(e) none of the above

(d) a net external force must be acting on the system

Explanation:

Momentum is given as the product of mass and velocity.

P = MV

According to Newton's second law of motion, " Force applied to a body (system) is directly proportional to the rate of change of momentum of the body (system) which takes place in the direction of the applied force (external force).

F ∝ΔMV

Therefore, If the total momentum of a system is changing, a net external force must be acting on the system.

(d) a net external force must be acting on the system