The doppler effect is the apparent change in caused by relative motion between the source of the wave and the observer.

The correct answer is frequency

The Doppler effect is characterized by the change in the sound frequency emitted by a source, perceived by an observer when there is relative movement. A typical example of the Doppler effect is the case of an ambulance with the siren on, during the approach or removal of an observer.

answer; /// i believe the correct answer is flash your high beams at them.or tap your horn;

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