A.No action is necessary，as the DSC control unit will automatically switch to the NBDP follow-on communications frequency
B.The operator should immediately set continuous watch on the radiotelephone frequency that is associated with the frequency band on which the distress alert was received
C.The Operator should immediately set continuous watch on VHF channel 70
D.The Operator should immediately set continuous watch on the NBDP frequency that is associated with the frequency band on which the distress alert was received

A.A compulsory vessel must carry at least two (2) licensed GMDSS operators
B.A compulsory vessel must satisfy certain equipment carriage requirements that are determined by where the vessel sails
C.A compulsory vessel must be able to transmit and respond to distress alerts
D.All of the above