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單項(xiàng)選擇題

A V-shaped ripple with the point of the V pointing upstream in a river may indicate a（）.

A.submerged rock，not dangerous to navigation
B.sunken wreck，not dangerous to navigation
C.towed-under buoy
D.All of the above

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單項(xiàng)選擇題

A vessel is（）when she is not at anchor，made fast to the shore，or aground.

A.underway
B.making way
C.dead in the water
D.a power driven vessel

單項(xiàng)選擇題

A structure，usually made of stone，or cement pilings，which extends from the bank at approximately right angles to the current is called a（）.

A.dike
B.revetment
C.cutoff
D.crib