Describe how you determine whether an object is in motion

To determine whether an object is in motion or not, you first
need to specify a reference point, because there's no such
thing as "real" motion, only motion relative to something.

Once you've named the reference point, you have to look at
the object at two different times.  Each time you look at it, you
measure its distance and direction from the reference point. 
If there's any difference in these measurements from one time
to the next, then the object has had average motion during the
period between the two observations.

That's the best you can do ... find average motion during some
period of time.  You can never definitely tell whether or not the
object ever stopped during that time.  But you can sneak up on
it by making the time period between the two observations shorter
and shorter.

Ithink the answer would be a