单项选择题
A.vessels ascending the river
B.vessels descending the river
C.all vessels ascending and descending the river
D.None of the above
单项选择题 A vessel continually lists to one side and has a normal rolling period. Which statement is TRUE?（）.
单项选择题 A vessel constrained by her draught may，in addition to the lights prescribed for power-driven vessels in Rule 23，exhibit where they can best be seen（）or a cylinder.
单项选择题 A vessel conducting mine clearing operations will show（）.