变压器的能量传递和平衡全靠磁势平衡。
参考答案：对
—How much is this football?
—You may _______ forty yuan for it. [ ]
A. spend
B. take
C. pay
D. cost
A．I= ，从上向下
B．I= ，从上向下
C．I= ，从下向上
D．I= ，从下向上
