单项选择题
On which type davit does the davit head stay at the same height?（）
A.Radial B.Sheath-screw C.Quadrantal D.Gravity
单项选择题 On what type of pump would you find an impeller?（）
单项选择题 On navigational aids，what does the light characteristic Fl(2+1) mean?（）
单项选择题 On cargo booms，preventers are（）.
