The critical temperature for superconductors is the temperature at which the electrical resistivity of a metal
A. drops to zero
B. increases to maximum limit
C. decrease to certain limit
D. none of the above

Hint:When a substance cools below a "critical temperature," it becomes a superconductor, which conducts electricity without resistance. Electrons can readily pass through the substance at this temperature. Ordinary conductors, even very good ones, are not superconductors.

Complete answer:
The electric resistance of a superconductor drops to zero relatively quickly below a temperature known as the "critical temperature." The material conducts current flawlessly with zero resistance. This is puzzling since when electrons pass through the material, the defects and vibrations of the atoms should generate resistance.

In a superconductor, however, the electric resistance is zero, notwithstanding the presence of defects and vibrations. As a result of the preceding information, we can deduce that the critical temperature is the temperature at which a conductor's resistivity becomes zero.The electrons in a superconductor combine into a single collective quantum wave known as the condensate.

Hence, the correct option is A.

Note: The electrical resistance of a superconductor is zero. This is why, even after the battery is disconnected, an electric current can circulate in a superconducting ring indefinitely! In MRIs, magnetic fields are formed in this manner. It may seem strange that a battery might generate a low-voltage electric current at the terminals of a superconducting network, given that the voltage should be zero due to the lack of electric resistance.