The Purpose of Rubbing a Mineral Across an Unglazed Porcelain

The purpose of rubbing a mineral across an unglazed porcelain slab, the streak plate, is:

A) to remove dirt and corrosion from the mineral so that its true color can be observed.
B) to observe the color of a mineral in its powdered form.
C) to remove the atoms of trace impurities that may exist within the structure of the mineral.
D) to determine if a streak will form on the mineral.