In a wheat stones bridge , three resistance P, Q and R are connected in three arms and the fourth arm is formed by two resistances $S_1$ and $S_2$ connected in parallel. The condition for the bridge to be balanced will be 

• A

  $\frac{P}{Q}=\frac{R\left(S_1+S_2\right)}{S_1{S}_2}$

• B

  $\frac{P}{Q}=\frac{R\left(S_1+S_2\right)}{2S_1{S}_2}$

• C

  $\frac{P}{Q}=\frac{R}{S_1+S_2}$

• D

  $\frac{P}{Q}=\frac{2R}{S_1+S_2}$