I have a new situation: John was sitting at point C near a railway track. Suddenly, he heard a train coming towards him from point A with constant sound of its horn. Eventually, it reached very close to him at point B. Then the train sped by him at point C where he was sitting, reaching point D after some time. Finally, it reached even farther from him at point E.  But I can use this background: Look at the police car in Figure below . The sound waves from its siren travel outward in all directions. Because the car is racing forward (toward the right), the sound waves get bunched up in front of the car and spread out behind it. As the car approaches the person on the right (position B), the sound waves get closer and closer together. In other words, they have a higher frequency. This makes the siren sound higher in pitch. After the car speeds by the person on the left (position A), the sound waves get more and more spread out, so they have a lower frequency. This makes the siren sound lower in pitch. A change in the frequency of sound waves, relative to a stationary listener, when the source of the sound waves is moving is called the Doppler effect . You’ve probably experienced the Doppler effect yourself. The next time a vehicle with a siren races by, listen for the change in pitch. For an online animation of the Doppler effect, go to the URL below.  What is an answer for this question: Which point would have lower frequency, point D or point E?
Answer:
point E