The area of cross-section of a copper wire is $4 \times 10^{-7} \mathrm{~m}^2$ and the electrons per cubic metre in copper is $8 \times 10^{28}$. If the wire carries a current of 6.4 A, then the drift velocity of the electrons (in $10^{-3} \mathrm{~m} \mathrm{~s}^{-1}$ ) is

• A

  $0.25$
• B

  $2.5$
• C

  $0.125$
• Correct answer
  D

  $1.25$

Hence, the correct answer is option: 'D'.