A scientific research boat sends out ultrasound pulses to locate fish below it. An ultrasound pulse is reflected back from a shoal of fish $6000\,\text{m}$ vertically beneath the boat. The following pulse is transmitted $2.0\,\text{s}$ after the echo from the earlier pulse is detected. The speed of ultrasound in water is $1500\,\text{m s}^{-1}$. What is the time gap between the two pulses?
- A$4.0\,\text{s}$
- B$6.0\,\text{s}$
- C$8.0\,\text{s}$
- D$10\,\text{s}$