单项选择题
A.easy and expensive
B.simple and certain
C.dangerous and stable
D.difficult and compulsory
单项选择题 In which year was the COSPAS-SARSAT system established?（）
单项选择题 When receiving a distress alert, the SAR authorities ashore and the ships in the vicinity of the ship in distress will（）in a coordinated search and rescue operation with the minimum delay.
单项选择题 In areas covered by Inmarsat HF can be used as an（）to satellite communications.