单项选择题
A.has been used B.have been used C.has used D.have used
All coast radio stations（）DSC installed for VHF and HF....
单项选择题All coast radio stations（）DSC installed for VHF and HF.
A.have B.had C.are having D.have had
The vessel has stayed in port for seven days because sh...
单项选择题The vessel has stayed in port for seven days because she needs（）.
A.repairs B.repairing C.repaired D.being repaired
（）by an ice-breaker, it is important to maintain a cont...
单项选择题（）by an ice-breaker, it is important to maintain a continuous listening watch on the appropriate VHF channel and to maintain a proper look-out for sound and visual signals.
A.which is assisted B.that assist you C.when you assisted D.when being assisting
