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.

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.