即能增大放大电路的输入电阻，又能使输出电流稳定的反馈为（）。
A．电压串联负反馈
B．电流串联负反馈
C．电压并联负反馈
D．电流并联负反馈
参考答案：B
The doctor _____ carefully and found nothing serious. [ ]
A. looked for him
B. looked him over
C. looked at him
D. looked him after
一个分数的分母扩大3倍，分子不变，则该分数应该是多少( )
A．保持不变
B．缩小3倍
C．缩小1/3
D．缩小1/30
