单项选择题
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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A.underway
B.making way
C.dead in the water
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A structure，usually made of stone，or cement pilings，which extends from the bank at approximately right angles to the current is called a（）.
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A spherical buoy may be（）.
A.numbered
B.lettered
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