单项选择题
The four standard light colors used for lighted aids to navigation are red，green，white，and （）.
A.purple
B.orange
C.blue
D.yellow
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The equator is（）.
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 -
单项选择题
Restricted areas at locks and dams are indicated by（）.
A.Flashing red lights upstream and fixed red lights downstream
B.Yellow unlighted buoys
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单项选择题
On a chart，the characteristic of the light on a lighthouse is shown as flashing white with a red sector.The red sector（）.
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
