细胞培养中血清的作用不包括（）

A.提供营养因子

B.防止细菌污染

C.促进细胞贴壁

D.促进细胞生长

E.酸碱缓冲能力

Prior to entering a davit-launched life raft, you should make sure that（）.

A.the life raft is well ventilated of excess carbon dioxide

B.all personnel have removed their life preservers to facilitate boarding

C.the door flap has been cut away and stowed inside the raft

D.All of the above