单项选择题
A.Distress messages should first be transmitted on 2182 kHz
B.Channel 16 (156.8 MHz) may be used for distress messages
C.If no answer is received on the designated distress frequencies，repeat the distress call on any frequency available
D.It is advisable to follow a distress message on 2182 kHz by two dashes of 10 to 15 seconds duration