单项选择题
What is meant by the term“luffing the boom”of a crane（）．
A.Stopping the boom B.Topping or lowering the boom C.Moving the boom left or right D.All of the above
单项选择题 What does “evel-luffing” accomplish during crane operations（）.
单项选择题 They（）loading at 1400 hours．
单项选择题 The net productivity per gang-hour can be improved through a decrease of（）.
