单选题What is meant by the term“luffing the boom”of a crane（）．AStopping the boomBTopping or lowering the boomCMoving the boom left or rightDAll of the above
单选题
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
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