Ultrasound is employed to chart the ocean floor. In one survey, the water depth is 1200 m. An ultrasound pulse is transmitted from the surface and, when it comes back to the ship, a second pulse is sent straight away. Over any interval of 8.0 s, five pulses are transmitted downwards from the surface. Calculate the speed of the ultrasound in water.
- A150 m s$^{-1}$
- B300 m s$^{-1}$
- C750 m s$^{-1}$
- D1500 m s$^{-1}$