Constantan wire is used in making standard resistances because its

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Alloys such as constantan and manganin are used in making standard resistance coils.
Reason $R:$ Constantan and manganin have a very small value of temperature coefficient of resistance.
In the light of the above statements, choose the correct answer from the options given below.

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 $\alpha = 4 \times 10^{-4} \ ^oC^{-1}$.

The temperature coefficient of resistance of a resistor wire is $0.00125\,^{\circ}C^{-1}$. At $300\,K$,its resistance is $1\,\Omega$. At what temperature in $K$ will its resistance be $2\,\Omega$?

Equal potentials are applied on an iron and copper wire of the same length. In order to have the same current flow in the two wires, the ratio $r_{\text{iron}} / r_{\text{copper}}$ of their radii must be (Given that specific resistance of iron = $1.0 \times 10^{-7} \, \Omega \cdot \text{m}$ and specific resistance of copper = $1.7 \times 10^{-8} \, \Omega \cdot \text{m}$)

What is conductivity? On which factors does its value depend? Write its unit and dimension.