单项选择题
A.always move slowly B.are in one place only C.sometime move sometime don‟t move D.are synchronizing with the movement of the earth
A.receive shore-to-ship distress alerting B.transmit ship-to-shore distress alerting C.receive message for public correspondence D.transmit and receive locating signals
A.can be overcome by using traditional techniques B.can not be overcome by satellite system C.can be overcome by satellite and digital techniques D.can not be tackled by any modern system