振荡电路产生自激振荡的振幅条件是：反馈到放大器输入端的电压幅值必须等于或大于放大器的原输入（）幅值。
A、电流
B、电阻
C、电容
D、电压
参考答案：D
现在有2000t石油要从油田运到炼油厂．如果每节油罐车的容量是80m3时，需要这样的油罐车______节（已知石油的密度是0.85×103kg/m3）．
Why not stay at home _____ the road is so slippery after the heavy snow? [ ]
A. since that
C. then that
B. now that
D. after that
