A police car with a siren of frequency 8 k H z is moving with uniform velocity 36 k m / h r towards a tall building which reflects the sound waves. The speed of sound in air is 320 m / s . The frequency of the siren heard by the car driver is : 8.50 k H z 8.25 k H z 7.75 k H z 7.50 k H z be obtained by heating?