The correct answer is: C. Mica is harder than copper.
Mica is a naturally occurring mineral that is composed of sheet silicates. It is a very good electrical insulator and is often used in commutators. Copper is a metal that is also used in commutators. It is a good conductor of electricity and is also relatively hard. However, mica is harder than copper. This means that mica is less likely to wear away than copper. This is important in commutators, as the commutator is a part of an electric motor that is constantly in contact with other parts. If the commutator were to wear away, it would not be able to function properly.
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