产生内部过电压的原因是什么？
参考答案：
产生内部过电压的原因有：
⑴投切空载变压器或空载线路；
⑵小电流接地系统发生单相接地出现间隙电弧；
⑶铁磁谐波。
—When did you find your pocket ________?
—It was when I was going to work by bus.[ ]
A. to pick
B. to be picked
C. picked
D. picking
20℃时50%NaOH溶液的密度是（）kg/m3。
A.1225
B.1325
C.1425
D.1525
