单项选择题
A(n)（）indicates that there is serious danger for vessel, crew and passenger.
A.distress alert
B.urgency message
C.safety message
D.routine message
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单项选择题
A(n)（）indicates that is serious and inmmediate danger for vessel, crew and passengers.
A.distress alert
B.urgency message
C.safety message
D.routine message -
单项选择题
Cirrocumulus belongs to（）.
A.low clouds
B.High clouds
C.Special clouds
D.medium clouds -
单项选择题
Altostratus belongs to（）.
A.Low cl;ouds
B.High clouds
C.Special clouds
D.medium clouds
