How to find when the particle changes direction?

As time increases, if the particle's velocity changes sign from positive to negative, or negative to positive, then it must have changed (opposite) direction on its linear path. As time increases on a graph of the particle's position versus time, it changes directions when position changes from increasing to decreasing, or from decreasing to increasing.

2. they have negligible mass.

Ibelieve that the correct answer is: c. along plate boundaries