Which particles are responsible for the flow of current in conductors?

The equivalent resistance between points $A$ and $B$ of the following electric circuit will be $.........$

Resistance of a metal wire of length $1\, m$ is $26\, \Omega$ at $20^{\circ} C$. If the diameter of the wire is $0.3\, mm$,what will be the resistivity of the metal at that temperature?

How many joules are there in one kilowatt-hour?

The resistivity of semiconductor materials is $..........$.

Of which physical quantity is the $joule/coulomb$ a unit?