单选题The Urgency Signal（）indicates that the station calling has a very urgent message to transmit concerning the safety of a ship or aircraft，or of some person on board or within sight.ASOSBXXXCTTTDPAN
单选题
The Urgency Signal（）indicates that the station calling has a very urgent message to transmit concerning the safety of a ship or aircraft，or of some person on board or within sight.
A
SOS
B
XXX
C
TTT
D
PAN
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