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A.vertical distance from the chart sounding datum to the ocean bottom，plus the height of tide
B.vertical distance from the chart sounding datum to the ocean bottom
C.average height of water over a specified period of time
D.average height of all low waters at a place

A.Gray lines on the uppermost inset chart
B.Red lines on the main body of the chart
C.In parenthesis on the lines of equal magnetic variation
D.Annual rate of change is not shown