Why is the average velocity of an electron at time $t$ equal to zero in the absence of an external electric field?

The magnitude of the drift velocity per unit electric field is known as . . . . . . .

When current flows through a conductor,then the order of drift velocity of electrons will be

$A$. The drift velocity of electrons decreases with the increase in the temperature of a conductor.
$B$. The drift velocity is inversely proportional to the area of cross-section of a given conductor.
$C$. The drift velocity does not depend on the applied potential difference to the conductor.
$D$. The drift velocity of an electron is inversely proportional to the length of the conductor.
$E$. The drift velocity increases with the increase in the temperature of a conductor.
Choose the correct answer from the options given below:

Drift velocity $V_d$ varies with the intensity of electric field $E$ as per the relation:

The potential difference between the ends of a straight conductor of length $20 \ cm$ is $16 \ V$. If the drift speed of the electrons is $2.4 \times 10^{-4} \ ms^{-1}$,the electron mobility in $m^2 \ V^{-1} \ s^{-1}$ is