A pulse of sound is sent out from the bottom of a boat. It passes through the water and is reflected by a shoal of fish. The sound returns to the boat after 1.2 s. The speed of sound in the water is $1500\,\text{m s}^{-1}$. Calculate how far below the bottom of the boat the shoal of fish is.
- A450 m
- B900 m
- C1800 m
- D3600 m