行胆囊超声检查的病人在检查前应禁食几小时（）
A.4～6小时
B.6～8小时
C.8～12小时
D.18小时
E.3小时
参考答案：C
The frightened thief hid himself in the dark corner of the yard, ______, staring at the open door. [ ]
A. quiet and cold
B. quietly and coldly
C. quiet and coldly
D. quietly and cold
引起事故发生的结构函数为：T=(X1X3+X2X4)(X1X2+X2X3+X4)，则导致发生事故的基本事件集合是______。
A．X1X2，X2X4B．X1X2X3，X1X3X4，X2X4C．X1X2X4D．X1X3，X2X3X4
