Answer
Ball A has a negative charge.
Ball B is neutral.
Ball C has a positive charge.
Ball D is neutral.
Work Step by Step
When a plastic rod is rubbed with wool, electrons move from the wool to the rod, so the rod becomes negatively charged.
When the rod touches ball A, then ball A becomes negatively charged.
Since balls B, C, and D are attracted to ball A, they could be positively charged or neutral.
Since balls B and D have no effect on each other, they must both be neutral.
Since ball B (which is neutral) is attracted to ball C, then ball C must be positively charged.
