A siren produces sound with frequency $1000\,\text{Hz}$. The siren is travelling at $20\,\text{m s}^{-1}$ towards an observer who is stationary. The speed of sound in air is $330\,\text{m s}^{-1}$. Which expression gives the frequency heard by the observer correctly?
- A$\dfrac{1000 \times 330}{330 + 20}$
- B$\dfrac{1000 \times 330}{330 - 20}$
- C$\dfrac{1000(330 + 20)}{330}$
- D$\dfrac{1000(330 - 20)}{330}$