What is the action that a GMDSS Radio Operator should take when a DSC distress alert is received? （）
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
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