Tom doesn’t want to take part in any school activities,________.

A．and David doesn’t too

B．and David doesn’t either

C．and so doesn’t David

D．and either does David

如图所示，电磁铁附近的小磁针处于静止状态，则电磁铁的A端是______极，当滑动变阻器的滑片向b端滑动时，电磁铁的磁性将______．