Answer: The correct answer is C.
Explanation:
Doppler effect is a phenomenon in which there is a relative motion between the source and the observer then there will be change in the frequency of the wave.
For example,when the source and the observer are moving each other then the frequency increases in this case. When the source and the observer are moving away from other then the frequency decreases in this case.
In the given problem, two fire trucks with sirens speeds towards and away from an observer. In this case, the fire the fire truck traveling away from the observer produces a lower than normal apparent siren frequency
Therefore, the correct answer is C.
