单项选择题
The positions and characteristics of lights and buoys shown within the port area are（）.
A.untrue B.impossible C.suspectful D.unimportant
单项选择题 The period of a lighted aid to navigation refers to the（）.
单项选择题 The nominal range of a light may be accurately defined as the maximum distance at which a light may be seen（）.
单项选择题 The maximum distance at which a light may be seen under the existing visibility conditions is called（）.
