Good pencil erasers are soft enough not ________paper but hard enough so that they crumble gradually when used.

A．by damaging

B．so that they damage

C．to damage

D．damaging

如图所示，虚线框内的磁场在均匀增大，直虚线经过圆心。则

A．在A点放上一正点电荷，点电荷将受到向右的电场力

B．A点的场强比B点的场强大

C．将一点电荷沿直线AB移动，电场力不做功

D．将一点电荷从A点静止释放，点电荷会沿电场线做圆周运动