A student writes the distance to the sea bed measured by a ship as d=vt, where t is the total time between sending an ultrasound pulse and receiving its echo. Which of the following best explains why this expression is incorrect?
(a) The speed of sound in water changes with depth, so d=vt is invalid.
(b) Water pressure reduces the wavelength of the pulse, making vt unsuitable.
(c) The ultrasound pulse travels the distance twice—down to the seabed and back up—so the actual depth is d=2vt.
(d) Refraction bends the path of the pulse, so the straight-line distance formula d=vt cannot be used.