Chapter 11, Problem 81RPP

If the car from Problem 11.80 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?

$\begin{array}{l}\text{a}\text{. 18,000 Hz}\hfill \\ \text{b}\text{. 19,000 Hz}\hfill \\ \text{c}\text{. 20,000 Hz}\hfill \\ \text{d}\text{. 21,000 Hz}\hfill \\ \text{e}\text{. 23,000 Hz}\hfill \end{array}$