单项选择题
Sometimes lights are（）by fog，haze，dust，smoke，or precipitation which may be present at the light，or between the light and the observer，and which is possibly unknown by the observer.
A.obscured
B.darkened
C.greyed
D.delighted
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Pilotage in this country is（）.
A.Free of charge
B.Important
C.Required
D.Unnecessary -
单项选择题
Mariners should ensure correct（）of aids to navigation during twilight periods when some lighted aids to navigation are lit while others are not.
A.identification
B.distinction
C.discrimination
D.clearly seeing -
单项选择题
If the winch should fail while you are hauling in the anchor，what prevents the anchor cable from running out（）.
A.Chain stopper
B.Devil's claw
C.Hawse ratchet
D.Riding pawl
