Answer:
Answer B. Adding carts increases the mass and increases the total energy in the system.
Step-by-step explanation:
By adding carts, the mass of the system is larger, and therefore, both the potential energy and the kinetic energy of the system will increase, thus contributing to larger final velocities as the carts roll down the tracks.
The correct answer is therefore the one shown in answer B:
Adding carts increases the mass and increases the total energy in the system.