题目

阅读理解。 Mr. Shaw wrote a lot of stories and people in the world knew him well. He didn’t like the arrogant(傲慢的) rich(富有的) men and often laughed at them. So people liked him very much. A lot of men hoped to know Mr. Shaw. One day a famous singer asked him to her party. When Mr. Shaw came in, all the people stood up except a young man. The young man’s father was one of the richest men in England and he was studying in Paris(巴黎). He thought he knew much more than anyone else in the room. He talked a lot and tried to show he was a know-all(自以为无所不知的物理课上，老师做了一个奇妙的“跳环实验”。如图，她把一个带铁芯的线圈I、开关S和电源用导终连接起来后.将一金属套环置于线圈L上，且使铁芯穿过套环。闭合开关S的瞬间，套环立刻跳起。某司学另找来器材再探究此实验。他连接好电路，经重复试验，线圈上的套环均末动。对比老师演示的实验，下列四个选项中.导致套环未动的原因可能是 A.线圈接在了直流电源上. B.电源电压过高. C.所选线圈的匝数过多， D.所用套环的材料与老师的不同