56（）

A．some kind of disaster a person experienced in his childhood
B．a vague sense that something will happen and it does happen later
C．the prediction of the death of a close friend
D．uncertainty about an impending accident

A．half-brain sleep is found in all kinds of birds
B．half-brain sleep is characterized by accelerated brain waves
C．birds can control their half-brain sleep consciously
D．birds always sleep with the whole of their brain at rest