双进双出钢球磨煤机在研磨的同时，煤粉被（）干燥，磨好的煤粉由干燥剂气流带出筒体。

The signal man assisting a crane operator has his arm extended with his fingers closed and thumb pointing upward. This is the signal to（）.

A.raise the boom

B.raise the hook

C.raise the boom and hook

D.check the boom stop