When an ambulance passes by, the siren’s pitch changes. The sounds becomes higher-pitched as it approaches, and then the pitch lowers as it gets further away. This change in pitch is known as the Doppler effect. What causes this change in pitch? answers:
A: The waves emitted by an object moving toward an observer are squeezed, and frequency appears to increase.
B: The waves emitted by an object moving away from an observer are squeezed, and frequency appears to increase.
C: The waves emitted by an object moving toward an observer are stretched, and frequency appears to decrease.
D: The waves emitted by an object moving toward an observer are stretched, and frequency appears to increase.