A siren at rest emits a sound of frequency f) The speed of sound in air is v. The siren moves away from an observer at rest relative to the siren in a straight line with constant speed vs. siren @ IF ~7 Vs @ observer The observer measures a frequency lower than f0 because the A. speed at which the sound moves relative to the observer is v vs. B. speed at which the sound moves relative to the observer is vS v. C. wavelength measured by the observer is smaller by a factor E. V D. wavelength measured by the observer is greater by a factor E. v