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（）.
单项选择题 A great circle track provides the maximum saving in distance on（）.
单项选择题 A gnomonic projection is based on a(n)（）.
