An oil drop, carrying six electronic charges and having a mass of $1.6\times {10}^{-12}\mathrm{g}$, falls with some terminal velocity in a medium. What magnitude of vertical electric field is required to make the drop move upward with the same speed as it was formerly moving downward with? Ignore buoyancy.

• A

  ${10}^5{\mathrm{NC}}^{-1}$

• B

  ${10}^4{\mathrm{NC}}^{-1}$

• C

  $3.3\times {10}^4{\mathrm{NC}}^{-1}$

• D

  $3.3\times {10}^5\cdot {\mathrm{NC}}^{-1}$