A Source Emitting Waves of Constant Wavelength Approaches an Observer

Multiple Choice

A source emitting waves of constant wavelength approaches an observer, then passes the observer, and recedes into the distance-all at a constant speed. The wavelength of the waves the observer measures from this source

A) remains constant.
B) gradually becomes longer and longer as the source approaches the observer and then becomes even longer as the source recedes from the observer.
C) gradually becomes shorter and shorter as the source approaches the observer and then gradually becomes longer and longer as the source recedes from the observer.
D) is shorter (and constant) as the source approaches the observer and then suddenly becomes longer (and constant) as the source recedes from the observer.