On a distance vs time graph the line of an object at rest is a

A point. A line would show progressing distance over passing time. Since the object is not going anywhere it would just be a point. I would also assume that this point would be sitting at the 0 on the distance axis. (this assumes Distance is the x axis)

If the x-axis is time and the y-axis is distance you will get a horizontal line because time will be passing but the distance will (Y value) constant (at rest).

pure substance: iron,copper

impure substance: water,charcoal

the answer for sodium is 0.186

the answer for rubidium is 0.251