You see a vessel carrying a black diamond which indicates a ().A.vessel towing asternB.bar

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You see a vessel carrying a black diamond which indicates a ().

A.vessel towing astern

B.barge pushed ahead

C.vessel towing astern only when the length of her tow exceeds 200 meters

D.this signal does not exist in international waters

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热搜:a是b的充分条件,b是a的必要条件吗