Sound from a ship is reflected back from a cliff. A sailor on the ship hears an echo $4.0\,\text{s}$ after the sound is emitted. The speed of sound in air is $320\,\text{m s}^{-1}$. How far away is the ship from the cliff?
- A$160\,\text{m}$
- B$640\,\text{m}$
- C$1280\,\text{m}$
- D$2560\,\text{m}$