A FICTIVE LINE DISPLAYED ON THE RADAR SCREEN SEPARATING THE FAIRWAY FOR INBOUND AND OUTBOUND VESSELS SO THAT THEY CAN SAFELY PASS EACH OTHER defines（）.

A.at all times
B.in the bottom return mode
C.in the volume reverberation mode
D.only when there is no current

A.speed being made good
B.speed through the water
C.the set of the current
D.the depth of the water

A.velocity of the current
B.bottom characteristics
C.depth of the water
D.speed over the ground