单项选择题

A polyconic projection is based on a （）.

A.plane tangent at one point
B.cylinder tangent at one parallel
C.cone tangent at one parallel
D.series of cones tangent at selected parallels

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单项选择题

A.when at anchor
B.only when making way
C.at any time when underway
D.only when the identifying lights are not being shown

单项选择题

A.aground
B.at anchor
C.not under command
D.being towed

单项选择题

A.average wind conditions
B.tidal currents
C.magnetic variation
D.average limits of field ice