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
单项选择题 A vessel is（）when she is not at anchor，made fast to the shore，or aground.
单项选择题 A structure，usually made of stone，or cement pilings，which extends from the bank at approximately right angles to the current is called a（）.
单项选择题 A spherical buoy may be（）.
