An aircraft travels with velocity $v_s$ straight away from a stationary observer. The aircraft produces a sound at a steady frequency. The speed of sound in air is $v$. The frequency of the sound received by the observer on the ground is $500\,\text{Hz}$. The aircraft’s speed is then increased so that it moves away from the observer with a larger velocity. The observer now hears a sound with frequency $250\,\text{Hz}$. Which expression gives the aircraft’s new velocity?
- A$2(v + v_s)$
- B$\frac{v + v_s}{2}$
- C$2v_s + v$
- D$2v_s - v$