What Is a Rotation Curve

What is a rotation curve?

A) A graph showing how orbital velocity depends on distance from the center for a spiral galaxy
B) A precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
C) A curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy
D) A graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis