For which material does resistivity decrease with an increase in temperature?

In the given circuit,the lengths of $AB$ and $BC$ are equal. The radius of $AB$ is three times that of $BC$. What is the ratio of the potential difference across $AB$ to that across $BC$?

The length of a given cylindrical wire is increased by $100 \%$. Due to the consequent decrease in diameter, the change in the resistance of the wire will be .................. $\%$

The power of a heater is $500 \ W$ at a temperature of $800 \ ^oC$. What will be its power at $200 \ ^oC$ in $W$? Given $\alpha = 4 \times 10^{-4} \ ^oC^{-1}$.

Silicon and copper are cooled from $300 \ K$ to $100 \ K$. What happens to their specific resistance (resistivity)?

The temperature coefficient of resistance at $0\,^{\circ}\text{C}$ is $\alpha = 0.00125\,^{\circ}\text{C}^{-1}$. At a temperature of $25\,^{\circ}\text{C}$,the resistance of a conductor is $1\,\Omega$. Find the temperature at which the resistance becomes $1.2\,\Omega$.