单项选择题
The transmission of a（）indicates that a ship is in distress and requires immediate assistance.
A.urgent traffic B.distress alert C.routine traffic D.NAVTEX message
单项选择题 The GMDSS is composed of（）.
单项选择题 MSI is broadcast via（）.
单项选择题 The existing terrestrial radio system has a number of basic flaws which（）.
