Two cars A and B are moving away from each other in opposite directions. Both cars move with a speed of 20 m/s-1 with respect to the ground. If an observer in car A detects a frequency 2000 Hz of the sound coming from car B, what is the natural frequency of the sound source in car B? (speed of sound in air = 340 m/s-1)
a. 2250 Hz
b. 2060 Hz
c. 2300 Hz
d. 2150 Hz