下列说法中不正确的是( )
A:电灯和电铃都应用了电磁铁
B:发电机工作时将机械能转化为电能
C:磁场和磁感线是客观存在的
D:电动机是利用电磁感应现象制成的
You'd better ask a ______, such as your aunt or sister, to look after your children while you are away.
如图,抛物线y=x2+bx+c与x轴交于A(﹣1,0)和B(3,0)两点,与y轴交于点C,对称轴与x轴交于点E,点D为顶点,连接BD、CD、BC.