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 wet cargo refers to（）.
单项选择题 A wildcat ia a（）.
单项选择题 A wind has caused a difference between drafts starboard and port. This difference is（）.
