The Observed Change in Wavelength Due to the Doppler Effect

Multiple Choice

The observed change in wavelength due to the Doppler effect occurs

A) whenever the light source is moving with respect to the observer (regardless of direction) .
B) only when the light source has a radial velocity (toward or away from the observer) .
C) only when the temperature of an object changes.
D) only when the light source has a proper motion (across the line of sight) .