$(a)$ Resistivity of iron is $10 \times 10^{-8} \; \Omega m$ and of mercury is $94 \times 10^{-8} \; \Omega m$. Which among iron and mercury is a better conductor? Give reason.
$(b)$ Resistivity of a material $A$ is in the range of $10^{10}-10^{14} \; \Omega m$ while that of $B$ is in the range of $10^{-6}-10^{-8} \; \Omega m$. Which of the two will behave as an insulator and why?

(A) Iron is a better conductor. The reason is that electrical conductivity is inversely proportional to resistivity. Since the resistivity of iron $(10 \times 10^{-8} \; \Omega m)$ is significantly lower than that of mercury $(94 \times 10^{-8} \; \Omega m)$,iron allows electric current to flow more easily.
$(b)$ Material $A$ will behave as an insulator. Insulators are materials that offer very high resistance to the flow of electric current,which corresponds to a very high resistivity. Since the resistivity of material $A$ $(10^{10}-10^{14} \; \Omega m)$ is extremely high compared to material $B$,it acts as an insulator.