改善结晶器传热的目的是减少气隙热阻。

If the coxswain of your lifeboat gives the command HOLD WATER you should（）.

A.complete the stroke，raise your oar slightly，swinging the oar slightly forward，and place it in the boat

B.lift the oar in a vertical position

C.complete the stroke and hold the oar out of the water

D.dip the blade of your oar into the water vertically and hold it perpendicular to the keel line