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

• A
  $3.6 \times 10^{-6}$
• B
  $2.4 \times 10^{-6}$
• C
  $2 \times 10^{-6}$
• D
  $3 \times 10^{-6}$