The correct answer is A. distance scale.
The pole distance is the distance between the two poles of a magnet. It is measured in meters or centimeters.
A force scale is a device used to measure the force of an object. It is measured in Newtons.
A mass scale is a device used to measure the mass of an object. It is measured in kilograms.
A time scale is a device used to measure the time of an event. It is measured in seconds.
