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 circles

（）是毛细血管的生理功能。

A、输血、输液

B、测量脉搏

C、物质交换

D、气体交换