Rubber rods charged by rubbing with cat fur repel each other. Glass rods charged by rubbing with silk repel each other. A rubber rod and a glass rod charged respectively as above attract each other. A possible explanation is that .

(A) All rubber rods always have an excess of positive charge on them.
(B) A rubber rod and a glass rod charged this way have opposite charges on them.
(C) Any two glass rods charged this way have opposite charges on them.
(D) Any two rubber rods charged this way have opposite charges on them.
(E) All glass rods always have an excess of negative charge on them.