Final answer:
The Doppler effect causes a siren's pitch to increase as it approaches because the siren's movement decreases the wavelength of the sound waves, resulting in a higher frequency and pitch perceived by the observer.
Step-by-step explanation:
The Doppler effect explains why a siren's pitch increases as it approaches you because the siren moves closer to the sound waves it has already produced, which decreases their wavelength. This decrease in wavelength results in an increase in the frequency of the sound waves, which our ears interpret as a higher pitch. Hence, the correct answer is B. The siren moves closer to sound waves it has already produced, decreasing their wavelength.
As an ambulance with a siren sounding approaches you, the sound waves it emits are compressed due to the vehicle moving toward you. This compression leads to a higher frequency of sound waves reaching your ears, which is why the pitch sounds higher. As the ambulance moves away, these waves are spread out, the wavelength increases, and the frequency decreases, which is why the pitch sounds lower.