Four lightweight balls A, B, C, and D are suspended by threads.
Ball A has been touched by a plastic rod that was rubbed with wool. When the balls are brought close together, without touching, the following observations are made:
• Balls B, C, and D are attracted to ball A.
• Balls B and D have no effect on each other.
• Ball B is attracted to ball C.
What are the charge states (glass, plastic, or neutral) of balls A, B, C, and D? Explain.