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)