Yang Li, a five-year-old girl living next door to me, very clever. Because she is not old enough to go to the school, she often asks me to teach herself to read, write and draw. She studies hardly and soon she learns a lot. One evening last month I was doing my English homeworks when she came to my house. She asked me how I learned English. “You would find English a bridge for so much knowledge,” I said. She then wanted to learn English, either. She has learned many English words so far but her parents are very grateful to me.

如图，闭合开关S，且将滑动变阻器的滑片P向右移动时，图中电磁铁

A．A端是N极，磁性增强

B．B端是N极，磁性减弱

C．A端是S极，磁性增强

D．B端是S极，磁性减弱