An（）indicates that there is imminent risk for navigation.

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

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

单项选择题（）is a high layer of cloud in the front of small flakes or cauhflower,white in colour with no dark shadows in between.

A.Cirrostratus
B.Cirrus
C.Cirrocumulus
D.Altostratus