单项选择题
A.the primary great circle of the Earth perpendicular to the axis B.the line to which all celestial observations are reduced C.the line from which a celestial body's altitude is measured D.All of the above
A.Flashing red lights upstream and fixed red lights downstream B.Yellow unlighted buoys C.Signs and/or flashing red lights D.Red daymarks upstream and green daymarks downstream
A.Indicates the limits of the navigable channel B.Indicates a danger area C.Is used to identify the characteristics of the light D.Serves no significant purpose