木板桩先插后打法优点是易于（）。
参考答案：合拢
As the old Chinese saying goes, only through reading thousands of books one can write ___.
A．fluently
B．correctly
C．energetically
D．professionally
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