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（）.
A.dike B.revetment C.cutoff D.crib
单项选择题 A spherical buoy may be（）.
单项选择题 A special purpose buoy shall be（）.
单项选择题 A special mark (yellow buoy)，if lighted，may exhibit which light rhythm?（）
