单项选择题
A.SOS three times
B.Additional transmissions to aid direction finding
C.E for one minute，followed by T for one minute
D.Two tones，10-15 seconds each
单项选择题 A message giving warning of a hurricane should have which prefix when sent by radiotelephone?（）
单项选择题 A Coast Guard radiotelephone message about an aid to navigation that is off station is preceded by the word（）.
单项选择题 A radiotelegraph message commencing with XXX has the same meaning as a message sent by the radiotelephone commencing with（）.