I think it can be A or D Hope this helps.
Answer:
A reference point.
Explanation:
Reference point:
A reference point is a point which is used to determine weather the object is in motion or not. The moving object is compared with the reference point.
An object is in the state of motion when its distance is changed relative to the other object or reference point.
In order to determine weather an object is moving or not a point of reference is needed.
Without reference point displacement can not be measured
Answer:
i am pretty sure it is compound
