There are four plates: and are charged plastic plates and is an electrically neutral copper plate. The electrostatic forces between some pairs of the plates are as follows: For the pair and do the plates repel or attract each other?
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since C is neutral and A attracts it, A must be +ve (same concept as why a rubbed comb attracts neutral piece of paper). So B is -ve and D is +ve. So force betn B and D is attractive.