道氏理论对大形势的判断有较大的作用，但对于每日每时都在发生的小波动显得无能为力。( )
参考答案：对
解析： 略
—Look,the girl at the gate _____ be my sister. She is always singing there every day.
—No. It _____ be her. She is dancing in the room now. [ ]
A. must, can
B. could, couldn't
C. must, can't
D. might, must
A．变压器输出端所接电压表的示数为20V
B．变压器的输出功率为200W
C．变压器输出端的交流电的频率为50Hz
D．穿过变压器铁芯的磁通量变化率的最大值为20/n2Wb/s
V
/n2Wb/s