18. Materials that block electric current from flowing are called D. copper C. metal A. insulators B. convectors 19. Why are electric wires usually made up of copper? A. Copper is a good insulator. B. Copper is a good conductor. C. Copper is a poor insulator, D. Copper is a poor conductor 20. Which of the following is an insulator? A. iron B. rubber C. water D. aluminum