Answer: (C) the electrons in the glass completely fill its valence levels and can’t shift from one level to another to transport charge through the glass.
Why: The electrons in an insulator such as glass completely fill the valence levels. Since only two electrons, one spin-up and one spin-down, can be in any level, the valence can't accommodate any shift in electrons from one level to another. The electrons can't respond to any electric fields and they can't conduct electricity.
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