Here are the answers to your questions regarding the Doppler effect:
Question 1: The Doppler effect is best known as a change in the _______ of a wave as the source is moving with respect to the observer.
Answer: A. frequency
Question 2: There is ___________ shift in sound frequency for observers towards whom the source is approaching.
Answer: B. an upward
Question 3: Which statement explains why the sound of an ambulance suddenly drops as it passes and travels away from you?
Answer: A. The distance between sound waves is increased.
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