问题：

[单选] How should you signal the crane operator to lower the boom and raise the load?（）.

A . Extend arm and point finger in the direction to move the boom
B . Extend arm with the thumb pointing downward and flex fingers in and out
C . With f rearm vertical and forefinger pointing up,move hand in small horizond circles
D . With arm extended downwards and forefinger pointing down,move hand in small horizontal circle