The Proper Motion of a Star Is

The proper motion of a star is:

A) the diameter of the circle through which the star appears to move in the sky each year, due to the motion of Earth.
B) the angle per unit time at which the star moves across our sky against the background stars.
C) the speed of the star in km/s, measured in a direction perpendicular to the line of sight from Earth to the star.
D) the speed of the star in km/s, measured along the line of sight from Earth to the star.