Which of the following is true about semi-conductors: A. Neither free electrons nor holes are considered charge carriers in a semi-conductor B. Exposing a crystal of a semiconductor to heat or light starts displacing valence electrons which then move throughout the crystal C. Gold and silver are important semiconductors D. A material is classified as a semiconductor if its resistance to the flow of electric current is too low to permit it to be called a conductor Mark for review (Will be highlighted on the review page)