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（）.

A.under existing visibility conditions
B.under perfect visibility
C.with ten miles visibility
D.with fifteen miles visibility

单项选择题The maximum distance at which a light may be seen under the existing visibility conditions is called（）.

A.nominal range
B.luminous range
C.charted range
D.geographic range

单项选择题The luminous range of a light takes into account the（）.

A.glare from background lighting
B.existing visibility conditions
C.elevation of the light
D.observer's height of eye

单项选择题Restricted areas at locks and dams are indicated by（）.

A.flashing red lights upstream and fixed red lights downstream
B.yellow unlighted buoys
C.signs and/or flashing red lights
D.red daymarks upstream and green daymarks downstream

单项选择题Preferred channel buoys indicate the preferred channel to transit by（）.

A.odd or even numbers
B.the color of their top band
C.the location of the buoy in the channel junction
D.the buoy's light rhythms