A box is released from rest and slides down a ramp in a closed system. Even though the box speeds up, its kinetic energy at the bottom of the ramp is less than the potential energy that it had at the top of the ramp. What best explains why the box’s final kinetic energy is less than its initial potential energy?

In a closed system, kinetic energy is always less than potential energy.
Some potential energy was converted into thermal energy due to friction.
The acceleration due to gravity became smaller as the box slid down the ramp.
In a closed system, total energy is constantly being lost to outside objects.