861. If a sound signal is emitted from the oscillator of a fathometer, and two seconds elapse before the returning signal is picked up, what depth of water is indicated?

The fathometer's two-second signal return time indicates a depth calculated using the formula Depth = (speed of sound x time) / 2. Given a standard speed of sound in seawater, this yields a depth of approximately 824 fathoms, making option D the correct answer.