A car horn has a rest frequency of 600 Hz. A stationary observer measures the sound to be 660 Hz just before the car passes and 550 Hz immediately after it passes. The speed of sound is 340 m/s. Which of the following values is closest to the speed of the car?
a) 34m/s
b) 15m/s
c) 31m/s
d) 28m/s