_____ distances prolong the time-of-flight and ______ distances shorten the time-of-flight.

• A. Greater distances prolong; Smaller distances shorten
• B. Not related
• C. Time-of-flight is inversely proportional to depth
• D. Distances do not affect time-of-flight

Answer: (A)

Time-of-flight grows with the distance the wave travels when the speed is fixed. In a round-trip measurement, the pulse covers 2 times the depth, so time = distance/speed (or t ≈ 2d/v). That means longer distances take longer to return, and shorter distances return sooner. So greater distances prolong the time-of-flight and smaller distances shorten it. The other ideas—no relation, inverse with depth, or no effect—conflict with the straightforward, direct proportionality between distance and travel time in a uniform medium. For example, doubling the depth roughly doubles the travel time if the speed stays the same.