A sound pulse is generated beneath the base of a boat. It passes through the water and is reflected by a shoal of fish. After $1.2\ \text{ s}$, the sound returns to the boat. The speed of sound in the water is $1500\ \text{m s}^{-1}$. How far below the bottom of the boat is the shoal of fish?
- A$450\ \text{m}$
- B$900\ \text{m}$
- C$1800\ \text{m}$
- D$3600\ \text{m}$