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]
a. 8.50 kHz
b. 8.25 kHz
c. 7.75 kHz
d. 7.50 kHz