To keep injured survivors warm in the water after abandoning a ship，they should（）.

A.be placed in the middle of a small circle formed by the other survivors in the water

B.float on their backs with their arms extended for maximum exposure to the air

C.remove their life preservers and hold on to the uninjured survivors

D.sip seawater at intervals of fifteen minutes

请根据所学的相关知识点完成下列填空：

自由组合定律告诉我们：在减数第（）次分裂的（）期，（）染色体上的（）基因自由组合。