11. Which of the following statements is correct ? (A) The conductivity of metal raises with the increase of temperature (B) the addition of boron into the silicon forms the n-type semiconductor (C) the decrease of temperature enhances the conductivity of semiconductors (D) because both valence band and conduction band are full, the electrons can not move so that the insulator is not conductive (E) because all the valence electrons of carbon form the covalent bonds, the diamond is not conductive