A source of sound sends out waves with a fixed frequency. Relative to a stationary observer, the source travels in a straight line at a constant speed. Which source velocity produces the lowest observed frequency?
- Aspeed $5\,\text{m s}^{-1}$, away from observer
- Bspeed $10\,\text{m s}^{-1}$, away from observer
- Cspeed $15\,\text{m s}^{-1}$, towards observer
- Dspeed $20\,\text{m s}^{-1}$, towards observer