流行病学研究的主要用途是（）。

A.进行统计学检验

B.探讨病因与影响流行的因素及确定预防方法

C.研究疾病的发生概率

D.研究疾病的死亡情况

E.研究疾病的临床表现

On a lifeboat equipped with Rottmer-type releasing gear，turning the releasing lever releases（）.

A.the painter

B.the after boat fall only if the boat is waterborne

C.both falls at the same time only if the boat is waterborne

D.both falls at the same time even if the boat has not reached the water