单项选择题
Radio beacon can be activated（）in case of distress.
A.only automatically B.only manually C.both automatically and manually D.only by remote control
单项选择题 If the beacon is mounted at a wrong place, it may（）by sea water, chemicals, exhaust and vibrations.
单项选择题 By using（）system a ship can send a distress message and know with certainty it will be received.
单项选择题 For COSPAS-SARSAT system,（）usually transmit initial COSPAS-SARSAT alert.
