How should you signal the crane operator to raise the boom?_________.
A.Extend arm with the thumb pointing up and flex the fingers in and out
B.Place both fists in front of the body with the thumbs pointing upward
C.With forearm vertical and forefinger pointing up，move hand in small horizontal circles
D.Extend arm with fingers closed and point thumb upward
正确答案：D
你应怎样指挥起货机手提升吊杆？伸开臂膀手指握紧拇指向上
