Consider dragging a box across a flat horizontal surface with friction at a constant, steady velocity. Is the mechanical energy of the box conserved?

1. Yes--there are no nonconservative forces present.
2. Yes--nonconservative forces are present but do no work.
3. Yes--nonconservative forces are present and do work, but the work done by some exactly cancels the work done by others.
4. No, mechanical energy is not conserved.