The term restricted visibility，when used in the Rules，refers to（）.

单项选择题The term PROLONGED BLAST refers to（）.

A.a blast of from five to seven seconds' duration
B.a blast of from five to six seconds' duration
C.a blast of from four to six seconds' duration
D.a blast of from six to seven seconds' duration

单项选择题The Rules state that risk of collision shall be deemed to exist（）．

A.Whenever two vessels approach from opposite directions
B.If the bearing of an approaching vessel does not appreciably change
C.Whenever a vessel crosses ahead of the intended track of another vessel
D.If one vessel approaches another so as to be overtaking

单项选择题The Rules state that a seaplane shall（）.

A.not be regarded as a vessel
B.in general，keep well clear of all vessels
C.proceed at a slower speed than surrounding vessels
D.when making way，show the lights for a vessel not under command

单项选择题The rules require which factor to be taken into account when determining safe speed（）.

A.The construction of the vessel
B.The experience of the vessel's crew
C.The location of vessels detected by radar
D.All of the above

单项选择题The Navigation Rules state that a vessel shall be operated at a safe speed at all times so that she can be stopped within（）.

A.The distance of visibility
B.1/2 the distance of visibility
C.A distance appropriate to the existing circumstances and conditions
D.The distance that it would require for the propeller to go from full ahead to full astern