Conductivity increases in the order of

$A$ wire of circular cross-section has an inner portion of radius $R$ made of a material of resistivity $\rho$ and is surrounded by an outer portion of thickness $R$ made of a material of double resistivity. Find the resistance of a length $l$ of such a wire.

The electric resistance of a certain wire of iron is $R$. If its length and radius are both doubled,then

Wires $P$ and $Q$ have the same resistance at room temperature. When heated,the resistance of $P$ increases and that of $Q$ decreases. We conclude that:

Two different conductors have the same resistance at $0\,^{\circ}C.$ It is found that the resistance of the first conductor at $t_1\,^{\circ}C$ is equal to the resistance of the second conductor at $t_2\,^{\circ}C.$ The ratio of the temperature coefficients of resistance of the conductors,$\frac{\alpha_1}{\alpha_2}$ is

The length of a hollow tube is $5\,m$,its outer diameter is $10\,cm$,and the thickness of its wall is $5\,mm$. If the resistivity of the material of the tube is $1.7 \times 10^{-8} \,\Omega m$,then the resistance of the tube will be: