单项选择题
A lighthouse can be identified by its（）.
A.painted color B.light color and phase characteristic C.type of structure D.All of the above
单项选择题 A lighted preferred-channel buoy may show a（）.
单项选择题 A light that has a light period shorter than its dark period is described as（）.
单项选择题 A light having characteristics which include color variations is defined as（）.
