The Proper Motions of Stars Are

Multiple Choice

The proper motions of stars are:

A) easy to measure for most stars on photographs taken six months apart.
B) noticeable to the unaided eye as a gradual shift of the constellations westward over the course of a month or two.
C) easy to observe with the unaided eye over the course of an hour or two.
D) difficult to measure, even on photographs taken several years apart for any but the nearest stars.