A toy drone produces a sound with constant frequency $800\,\text{Hz}$. The speed of sound in air is $330\,\text{m s}^{-1}$. The drone travels in a straight line directly towards an observer and then carries on in the same straight line directly away from the observer. The drone moves at constant speed. Calculate the velocity of the drone when the frequency of the sound detected by the observer is $850\,\text{Hz}$?
- Amagnitude $19\,\text{m s}^{-1}$, away from the observer
- Bmagnitude $21\,\text{m s}^{-1}$, away from the observer
- Cmagnitude $19\,\text{m s}^{-1}$, towards the observer
- Dmagnitude $21\,\text{m s}^{-1}$, towards the observer