（）is the distance east or west measured in degrees，minutes and seconds of arc on the equator between the prime meridian and the meridian passing through the place.

单项选择题A light having characteristics which include color variations is defined as（）.

A.switching
B.alternating
C.oscillating
D.fluctuating

单项选择题A light characteristic of composite group flashing indicates that there is a(n)（）.

A.sharp turn in the channel
B.narrowing in the channel at that point
C.junction in the channel
D.obstruction that must be left to port

单项选择题A lateral system buoy displaying a quick light（）.

A.should be passed close aboard on either side
B.indicates that special caution is required
C.is used at a channel bifurcation or junction
D.is painted with red and white vertical stripes

单项选择题A great circle track provides the maximum saving in distance on（）.

A.easterly courses in high latitudes
B.southerly courses in high latitudes
C.westerly courses in low latitudes
D.easterly courses in low latitudes that cross the equator

单项选择题A gnomonic projection is based on a(n)（）.

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