A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr 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 [2011]
(1) 8.50 kHz
(2) 8.25 kHz
(3) 7.75 kHz
(4) 7.50 kHz