The statement that must be true about the two postal delivery workers is "They each have a displacement of 0 miles."
Displacement is defined as the change in position from the starting point to the ending point. Since both workers travel from the post office, deliver mail, and return back to the post office, their starting and ending points are the same, resulting in a displacement of 0 miles.
The other statements are not necessarily true:
- They may not travel the same distance, as their routes could vary significantly.
- They do not have the same velocity, as velocity takes into account both speed and direction, which can differ based on their routes.
- They do not each travel a distance of 0 miles, as they certainly travel some distance to complete their routes before returning.
So, the correct answer is that they each have a displacement of 0 miles.