单项选择题
The points on the earth’s surface where the magnetic dip is 90° are（）.
A.Along the magnetic equator
B.Connected by the isoclinal line
C.The isopors
D.The magnetic poles
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The four standard light colors used for lighted aids to navigation are red，green，white，and （）.
A.purple
B.orange
C.blue
D.yellow -
单项选择题
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 -
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Restricted areas at locks and dams are indicated by（）.
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
