单项选择题
Exchange of answer-backs acts a confirmation that the messages（）at the destination.
A.have received B.have been received C.are received D.are being received
单项选择题 The beacon located on board aircraft is termed（）.
单项选择题 （）is capable of finding any 406MHZ beacons no matter where they are.
单项选择题 The COSPAS-SARSAT 406MHZ system was adopted as an element of the GMDSS by IMO in（）.
