D) Lower or higher depending on the speed of the van
Correct Answer: A
Solution :[a] When a police car or emergency vehicle was traveling towards you on the highway, as the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect- a shift in the apparent frequency for a sound wave produced by a moving source.
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