Choose the correct alternative:
$(a)$ Alloys of metals usually have (greater/less) resistivity than that of their constituent metals.
$(b)$ Alloys usually have much (lower/higher) temperature coefficients of resistance than pure metals.
$(c)$ The resistivity of the alloy manganin is nearly independent of/ increases rapidly with increase of temperature.
$(d)$ The resistivity of a typical insulator (e.g.,amber) is greater than that of a metal by a factor of the order of $(10^{22}/10^{23}).$

(A) Alloys of metals usually have greater resistivity than that of their constituent metals because the disordered structure of the alloy increases electron scattering.
$(b)$ Alloys usually have much lower temperature coefficients of resistance than pure metals,making them useful for standard resistors.
$(c)$ The resistivity of the alloy manganin is nearly independent of increase of temperature,which is why it is used to make standard resistors.
$(d)$ The resistivity of a typical insulator is greater than that of a metal by a factor of the order of $10^{22}$.