Which of the following resistivity ( $\rho$ ) $v/s$ temperature $( T )$ curves is most suitable to be used in wire-bound standard resistors?

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

The resistances of a wire at temperatures $t\,^oC$ and $0\,^oC$ are related by:

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 resistance of a coil is $4.2\, \Omega$ at $100\,^{\circ}C$ and the temperature coefficient of resistance of its material is $0.004\,^{\circ}C^{-1}$. Its resistance at $0\,^{\circ}C$ is ............. $\Omega$.

$A$ wire of resistance $R$ and radius $r$ is stretched until its radius becomes $r / 2$. If the new resistance of the stretched wire is $x R$,then the value of $x$ is $\qquad$