单项选择题 A special mark (yellow buoy)，if lighted，may exhibit which light rhythm?（）

单项选择题 A SEAMARK，I.E. A BUOY，INDICATING THE NORTH，EAST，SOUTH OR WEST FROM A FIXED POINT，E.G. A WRECK is（）.

单项选择题 A rock and sand structure extending from the bank of the river toward the channel is known as a（）.