4

Set 3: Central Ideas (Intermediate)

Explanation

Answer: D

PASSAGE

The following text discusses the Doppler Effect. The Doppler Effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. A common example is the change in pitch of a passing siren. As the vehicle approaches, the sound waves are compressed, creating a higher pitch. As it moves away, the waves are stretched, resulting in a lower pitch. Astronomers use this same principle to determine if stars are moving toward or away from Earth.

What causes the perceived change in pitch of a passing siren?

A. The loudness of the siren increasing
B. The compression or stretching of sound waves due to motion
C. The driver of the vehicle changing the siren settings
D. The echo of the sound bouncing off buildings✓ Correct

Detailed Explanation

Choice B is correct. The text explains: 'As the vehicle approaches, the sound waves are compressed... As it moves away, the waves are stretched.'

Key Evidence:

• "change in frequency... relation to an observer moving"

• "sound waves are compressed"

• "waves are stretched"

Why others are wrong: A (Confuses pitch with volume), C (Not mentioned), D (Not mentioned).

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