The Wheatstone bridge principle is used to measure the specific resistance $(S_1)$ of a given wire,having length $L$ and radius $r$. If $X$ is the resistance of the wire,then the specific resistance is: $S_1 = X \left( \frac{\pi r^2}{L} \right)$. If the length of the wire is doubled,then the value of the specific resistance will be:

• A
  $\frac{S_1}{4}$
• B
  $2 S_1$
• C
  $\frac{S_1}{2}$
• D
  $S_1$