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（）.
单项选择题 Altostratus belongs to（）.
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.
