Which pair of variables defines motion?

position (x) and time (t)

Explanation:

A body is said to be in motion when its position changes with time with respect to a stationary observer.

Following are the types of motion:

Uniform motion: When equal amount of distance is covered in equal intervals of time.

Non-Uniform motion: When unequal amount of distance is covered in equal intervals of time.

Motion can be of the following types as well:

Rectilinear motion: when object moves in a straight line.

Circular motion: when object moves in a curved path.

Periodic motion: when motion repeats itself in fixed intervals of time.

Thus, in order to define motion, only two variables are required: position and time. Measuring these variables can determine whether the object is in motion or not and the type of motion.

layer a is the exosphere and layer b is the thermosphere.   is the answer i think but i hope this .