When a galvanometer is shunted by a resistance $S$,its current capacity increases $n$ times. If the same galvanometer is shunted by another resistance $S^{\prime}$,its current capacity increases to $n^{\prime}$. The value of $n^{\prime}$ in terms of $n, S$ and $S^{\prime}$ is

• A
  $\frac{n+S}{S^{\prime}}$
• B
  $\frac{S(n-1)-S^{\prime}}{S^{\prime}}$
• C
  $\frac{(n+1) S}{S^{\prime}}$
• D
  $\frac{S(n-1)+S^{\prime}}{S^{\prime}}$