磁力线有哪些特点？
参考答案：
磁力线有如下特点：磁力线是不相交的闭合曲线。在磁体外部由N极指向s极，在磁体内部由s极指向N极。
— Tom is never late for work. Why is he absent today?
— Something _____ to him. [ ]
A. must happen
B. should have happened
C. could have happened
D. must have happened
“立嫡以长不以贤，立子以贵不以长”是哪一种王位继承制的基本原则。（）
A.兄终弟及制
B.父死子继制
C.嫡长子继承制
D.王子继承制
