Why should you not use emery cloth for polishing the commutator of an appliance motor? A. It wears the surface too much. B. Its dust conducts electricity. C. It fails to remove oily, discolored spots. D. Its material reacts chemically with copper.

The correct option is D. It is not ideal to use emery clothe to clean commutator because, particles of emery are conductive and they may short circuit the commutator bars. If the commutator becomes black, the best way to clean it is by wiping it with dry, lint free cloth. The motor could also be vacuumed using dry air.