Determined by the medium through which the sound travels.

• A. Echo arrival time
• B. Frequency
• C. Power
• D. Gain

Answer: (A)

Sound travels at different speeds in different media, and that speed governs how long an echo takes to come back. The echo arrival time is determined by the round-trip distance divided by the speed of sound in the medium, so the same distance will yield different times in air, water, or solids. This is why measuring how long the echo takes to return lets you infer distance, and why the medium matters for timing. Frequency, on the other hand, comes from the source and remains the same across media (though wavelength changes with speed). Power and gain describe amplitude or amplification and aren’t about how long the sound takes to travel.