Correct answer: A. Because of corrections for tide, sound speed, position and data processing
Explanation: Direct answer: A, corrections for tide, sound speed, position and data processing. Measured depth is not automatically the same as the depth printed on a chart. Tides change the water level, so observations may need a common tidal reference. Sound speed changes with temperature, salinity and pressure, affecting sonar travel time. The vessel’s position, instrument errors, motion and data-processing methods can also affect the result. Therefore A correctly identifies several reasons for differences. B is absurd: the seafloor does not disappear every second. C is wrong because charts do use a scale, and lack of scale is not the normal reason for a depth difference. D is wrong because ocean depths are not all equal. A charted value is a corrected, referenced and processed measurement, not a casual single reading. Memory cue: chart depth is measured depth plus corrections and reference.