If the car from Problem 78 is moving at 20 m/s toward the stationary car, which answer below is closest to the frequency of the reflected wave detected by the moving car from the stationary car?
(a) $18,000 \mathrm{Hz}$ (b) $19,000 \mathrm{Hz}$ (c) $20,000 \mathrm{Hz}$
(d) $21,000 \mathrm{Hz}$ (e) $23,000 \mathrm{Hz}$