An Astronomer, Examining Two Photographs of a Small Area of Sky

Multiple Choice

An astronomer, examining two photographs of a small area of sky taken 20 years apart, notices that some of the stars have moved slightly across the sky, relative to most of the other stars in the photographs. This displacement of the stars across the sky is an example of

A) proper motion.
B) Doppler shift.
C) precession.
D) radial velocity.