A police car with a siren of frequency $8$ kHz is moving with uniform velocity $36 \mathrm{km}\, \mathrm{hr}^{-1}$ towards a tall building which reflects the sound waves. The speed of sound in air is $320 \mathrm{m}\, \mathrm{s}^{-1}$. The frequency of the siren heard by the car driver is?
- A: $8.50$ kHz
- B: $8.24$ kHz
- C: $7.75$ kHz
- D: $7.50$ kHz