（）反映证券或组合的收益水平对市场平均收益水平变化的敏感性。

A.证券组合的期望收益率

B.β系数

C.证券间的相关系数

D.证券各自的权重

I____ asleep because it took me a long time to realize that the telephone was ringing.

A．could have fallen

B．should have fallen

C．must have fallen

D．mustn’t have fallen