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.nature of distress
B.type of assistance required
C.course and speed and time at which the information was recorded
D.A, B, C are all right

A.when two stations communicate with each other
B.in any radio system and at any time
C.when one ship sends a message to another station
D.for communication among some stations

A.VHF channels 16 and 13
B.Appropriate DSC distress and safety frequencies
C.INMARSAT TDM message channel
D.VHF channel 70 and channel 8