单项选择题
What type of stern tube bearing has the least friction（）.
A.Oil-lubricated bearings B.Lignum vitae C.Hard rubber D.Bronze bushings
单项选择题 What is a cofferdam（）.
单项选择题 What can cause a lack of oxygen in a chain locker（）．
单项选择题 Tonnage openings must be closed by means of（）.
