Which of the following has a negative temperature coefficient of resistance?

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

The resistance of a conductor at $15^{\circ}C$ is $16\, \Omega$ and at $100^{\circ}C$ is $20\, \Omega$. What will be the temperature coefficient of resistance of the conductor?

Two wires $A$ and $B$ are made up of the same material and have the same mass. Wire $A$ has a radius of $2.0 \ mm$ and wire $B$ has a radius of $4.0 \ mm$. The resistance of wire $B$ is $2 \ \Omega$. The resistance of wire $A$ is . . . . . . $\Omega$.

The variations of resistivity $\rho$ with absolute temperature $T$ for three different materials $X, Y$ and $Z$ are shown in the graph below. Identify the materials $X, Y$ and $Z$.

$A$ wire of resistance $3 \Omega$ is stretched to twice its original length. The resistance of the new wire will be (in $Omega$)