When an automobile moves towards a listener, the sound of its horn seems relatively A) low pitched B

Answer

B) High pitched. When the automobile moves toward the listener, the sound waves are compressed, so the observed frequency increases (Doppler effect), making the pitch sound higher.

Explanation

What the question is testing

This is a Doppler effect question. You are comparing the pitch (frequency) heard by a listener when the source of sound moves toward them.

What changes when the car moves toward you

As the horn (sound source) moves toward the listener, each successive wave crest is emitted from a position closer to the listener. That reduces the wavelength in front of the car.

Connecting wavelength to pitch

Sound speed in air is roughly constant for this situation, and it satisfies

$$v = f\lambda$$

If the wavelength $\lambda$ decreases while $v$ stays the same, the frequency $f$ must increase. Higher frequency means higher pitch.

Choosing the correct option

So the horn sounds higher pitched as the automobile approaches: option B.

Quick check with the Doppler formula (optional)

For a source moving toward a stationary listener,

$$f' = f\cdot \frac{v}{v - v_s}$$

Since $v - v_s < v$, the ratio is greater than 1, so $f' > f$ (higher pitch).