Cross-sectional area of a copper wire is equal to the area of a square of length $2 \ mm$. If this copper wire draws $8 \ A$ electric current,then find the drift velocity of free electrons. The number density of electrons in the copper wire is $8 \times 10^{28} \ m^{-3}$.

• A
  $1.56 \times 10^{-4} \ ms^{-1}$
• B
  $1.56 \times 10^{-2} \ ms^{-1}$
• C
  $3.12 \times 10^{-3} \ ms^{-1}$
• D
  $3.12 \times 10^{-2} \ ms^{-1}$