The unknown resistance in Kelvin’s double bridge is given by Where:\(Error\;{\rm{\Delta }} = \left[ {\frac{P}{Q} - \frac{p}{q}} \right]\)

• A. \(X = \left[ {\frac{P}{Q}S + \frac{{q{\rm{\Delta }}r}}{{p + q + r}}} \right]\)
• B. \(X = \left[ {\frac{P}{Q}S + \frac{q}{{p + q - r}}} \right]\)
• C. \(X = \left[ {\frac{P}{Q}S - \frac{{q{\rm{\Delta }}r}}{{p + q + r}}} \right]\)
• D. \(X = \left[ {\frac{P}{Q}S - \frac{q}{{p + q + r}}} \right]\)