单项选择题
A radio determination system based on the comparison of reference signals with radio signals reflected,or retransmitted from the position to be determined,is（）.
A.DF B.Radar System C.DSC System D.COSPAS-SARSAT
单项选择题 What information will a distress alert contain?（）
单项选择题 The ARQ mode should be applied（）.
单项选择题 It is required that ships proceeding at sea should keep continuous watch on（）.
