1. Which type of bond holds the molecules of the DNA double helix together?
(a) Covalent bond.
(b) Ionic bond.
(c) Einstein bond.
(d) Van der Waals bond.
2. In a p-type semiconductor, a hole is
(a) A region in the molecular structure where an atom is missing.
(b) An extra electron from one of the donor atoms.
(c) An extra positively charged particle in the molecular structure.
(d) A region missing an electron relative to the rest of the molecular structure.
3. The electrical resistance of a semiconductor may decrease with increasing temperature because, at elevated temperature, more electrons
(a) Collide with the crystal lattice.
(b) Move faster.
(c) Are able to jump across the energy gap.
(d) Form weak van der Waals bonds.