Why is it important to define a frame of reference?

A. An observer cannot be moving.
B. Motion must be defined relative to something.
C. It's impossible to measure motion.
D. A stationary observer is always moving.

Question

Why is it important to define a frame of reference?

 		A.	An observer cannot be moving.
 		B.	Motion must be defined relative to something.
 		C.	It's impossible to measure motion.
 		D.	A stationary observer is always moving.

michele_laino · Answer

hint:
there isn't the absolute motion, namely all motions are relative motions, so, in order to describe a motion mathematically, or in order to make some measurements about a certain motion, we need of a reference system