Final answer:
A trailer bouncing up and down slowly on the highway is likely nearly empty, as a heavy load would dampen such oscillations, and an empty trailer allows the suspension to move more freely.
Step-by-step explanation:
When you pass a freight truck with a trailer on the highway, and you notice that its trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty. A heavily loaded trailer would have less bounce due to the increased weight pressing down on the suspension system, creating more damping and less oscillation. In contrast, an empty or lightly loaded trailer has less force to counteract the springs in the suspension, resulting in more pronounced and slower bounces as the springs are not as compressed and can extend more freely.