单项选择题
（）are used for distress alerting as a rule.
A.SES/DSC/EPIRB B.SART/radiotelephone C.COSPAS-SARSAT D.both B and C
单项选择题 Mariners can use DSC for（）.
单项选择题 Equipment requirement vary according to（）.
单项选择题 The WWNWS establishes three types of navigational warning:（）.
