单项选择题
Our jumbo boom can（）20 tons.
A.lift B.rise C.elevate D.hoist
单项选择题 On which type davit does the davit head stay at the same height?（）
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?（）
