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
单项选择题 Restricted areas at locks and dams are indicated by（）.
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（）.
单项选择题 In very high latitudes，the most practical chart projection is the（）.
