Final answer:
A trailer observed to be bouncing slowly on the highway is likely nearly empty because the suspension is not weighed down and thus does not absorb road imperfections as effectively as when it is loaded heavily.
Step-by-step explanation:
When you see a trailer bouncing up and down slowly on the highway, it is likely that the trailer is nearly empty. The suspension of an empty trailer is less weighed down, which means it does not absorb bumps and dips in the road as smoothly as a heavily loaded trailer would. In contrast, a heavy load would keep the trailer's suspension more compressed, causing less bounce and providing a smoother ride as it would absorb the impact of the road more effectively.