单项选择题
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
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
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