单项选择题
The circular steel structure installed around the propeller of a towboat is the（）.
A.nozzle B.shroud C.strut D.hood
单项选择题 The bow thruster generally is ineffective at（）.
单项选择题 Structural bulkheads on a ship are usually（）.
单项选择题 Signs of racking stresses generally appear at the（）.
