Before entering the room,I was asked to show the soldier the________ of my suitcase.
A:position
B:shape
C:contents
D:instructions
方程x2﹣3x﹣4=0的两根之和为( )
在奥斯特实验中我们发现:在直导线周围放上小磁针,当导线中通电时,小磁针转动.这说明 ____,电流通过它产生的 ____对磁体施加力的作用,当电流方向改变时, ____方向改变.