单项选择题
In radiotelephone communications，the prefix PAN-PAN indicates that（）.
A.a ship is threatened by grave and imminent danger and requests immediate assistance
B.a calling station has an urgent message about the safety of a person
C.the message following the prefix will be about the safety of navigation
D.the message following is a meteorological warning
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单项选择题
Fog forms when the air（）.
A.is 50% water saturated
B.is 90% water saturated
C.temperature is greater than the dew point temperature
D.temperature is equal to，or below the dew point temperature -
单项选择题
Five or more short blasts on a vessel’s whistle indicates that she is（）.
A.in doubt that another vessel is taking sufficient action to avoid a collision
B.altering course to starboard
C.altering course to port
D.the stand-on vessel and will maintain course and speed -
单项选择题
After a vessel transmits the original distress message，what signal should be sent to allow ships to take RDF bearings?（）
A.SOS three times
B.Additional transmissions to aid direction finding
C.E for one minute，followed by T for one minute
D.Two tones，10-15 seconds each
