把两个大小相等、极性相反的信号加在差动放大电路的输入端称为差模输入。
参考答案：对
—When is Jack going to finish writing that program?
—I don't know ______ day in and day out.[ ]
A. He has been working
B. He has worked
C. He had been working
D. He was working
铁路道口普示标用以提醒通过道口的车辆和行人：注意火车，防止冲撞。（）
