A jet aircraft moves with speed $0.8v$, where $v$ represents the speed of sound. The aircraft is moving towards an observer at rest. The sound produced by the aircraft has a frequency of $100\,\text{Hz}$. Which frequency is heard by the observer?
- A$56\,\text{Hz}$
- B$180\,\text{Hz}$
- C$400\,\text{Hz}$
- D$500\,\text{Hz}$