A police car siren emits a 900 Hz tone as the car travels due east at 30 m/s. A motorist in another car is moving due west (directly toward the police car) at 20 m/s on the same straight road. The speed of sound in air is 343 m/s. What frequency does the motorist hear?
a) 9.86×102 Hz
b) 9.30×102 Hz
c) 1.04×103 Hz
d) 8.80×102 Hz