The correct answer is B: Glass. Glass is a good electrical insulator because it has very few free electrons and very high resistance, preventing current flow. Insulators are materials that oppose electrical current flow. Glass is made of silicon dioxide and other compounds that have tightly bound electrons. These electrons are not free to move, so glass has very high electrical resistance. This makes glass an excellent insulator - it prevents electrical current from flowing through it. Glass is commonly used as an insulator in electrical equipment, such as insulating supports, insulators on power lines, and insulating materials in electronic components. Good insulators have high resistance and few free electrons, opposite of good conductors.