SAT2物理上面的一道题An electrically charged,insulated metal rod is observed to attract a neutral pith ball and,after contact is made,to repel the ball,which of the folling sentence could be true.the rod's charge before and after the contact had the same sign为什么是这个

First,we can assume that rod's charge before the contract is Q,and after the contract,because the pith ball is electric neutrally,the Q electric quantity would balanced divide,both rod and pith ball would obtain 1/2Q electric quantity.So the rod's charge before and after the contact had the same sign.
In this question,the essence is the pith ball is neutral,so there is not added charged in this system.Therefore,the sign of the charge would not change.