A:{1,2,3}
B:{5,9}
C:{4,6}
D:{1,2,3,4,6}
——Must we stay in the classroom and finish all the homework?——No, you______. You can take it home.
条形磁体旁的小磁针静止时所指的方向如图所示,试标出条形磁体的N、S极和磁感线的方向及小磁针的N极.