Doppler Effect Steps:

• Heard frequency (404 Hz) exceeds source frequency (400 Hz), so car moves towards observer.
• Doppler formula for moving source, stationary observer: $f' = f \frac{v}{v - v_s}$ (minus sign for approach).
• Substitute $f' = 404$, $f = 400$, $v = 340$: solve $$.
• Calculation yields $v_s \approx 37$ m/s. Why B is correct:
• Matches Doppler formula for source approaching at 37 m/s, producing observed higher frequency. Why the others are wrong:
• A: Movement away decreases frequency below 400 Hz.
• C: Movement away decreases frequency below 400 Hz.
• D: 42 m/s towards produces frequency shift larger than 404 Hz.

Final answer: B