Which of the following relative motions between a siren and an observer will cause the observer to hear the SAME frequency as the siren’s rest frequency?
(a) The observer moves straight toward the stationary siren at constant speed.
(b) The siren moves straight toward the stationary observer at constant speed.
(c) The siren moves at constant speed along a path that is perpendicular to the line joining it and the observer.
(d) The siren moves at constant speed along a path that makes a 45∘ angle with the line joining it and the observer.