题目

  My 10-year-old Donna said, “Mom, I made a new friend at school today. Can she come over tomorrow?” Donna was a shy girl and I wanted her to make some friends to bring her out of her shell.   “Sure, honey, that sounds great.” I said, thinking back to my own best friend, Lillian. We lived across the street from each other in Washington Heights, New York. We met at the age of 10, too. Like my daughter, I was shy, but Lillian drew me out. She was one of the friendliest people in school, with shiny black hair and a mile-wide smile.    In senior high school, Lillian went on a如图所示，两个相连接的金属环用同样规格的导线制成，大环半径是小环半径的4倍，若穿过大环的磁场不变，穿过小环中磁感应强度随时间的变化率为k时，其路端电压为U；若穿过小环中磁场不变，而穿过大环中磁感应强度随时间的变化率为2k时，其路端电压为（ ）A．UB．2UC．4 UD．8U