4. What is a superconductor? (a) a wire that is cooled to near absolute zero (b) a material through which there is no resistance (c) a wire either made of silver or that can conduct electricity as well as silver (d) the wires drawn on computer chips and other micro-circuitry

The answer is (b).<br /><br />A **superconductor** is a material that exhibits zero electrical resistance when cooled below a critical temperature. Option (a) is partially correct in that many superconductors need to be cooled to very low temperatures, but simply cooling a wire to near absolute zero doesn't automatically make it a superconductor. Option (c) is incorrect; silver is a good conductor but not a superconductor. Option (d) is also incorrect; the wires on computer chips are typically made of copper, aluminum, or other good conductors, but not necessarily superconductors.<br />