In CGS system, the Young's modulus of a steel wire is $2\times {10}^{12} \mathrm{dyne}/{\mathrm{cm}}^2$. To double the length of a wire of unit cross-sectional area, the force required is

• A

  $4\times {10}^6$ dynes

• B

  $2\times {10}^{12}$dynes

• C

  $2\times {10}^{12}$newtons

• D

  $2\times {10}^8$dynes