A small conducting sphere of radius $r$ is lying concentrically inside a bigger hollow conducting sphere of radius $R.$ The bigger and smaller spheres are charged with $Q$ and $q (Q > q)$ and are insulated from each other. The potential difference between the spheres will be
  • A$\frac{1}{{4\pi { \in _0}}}\left( {\frac{q}{r} - \frac{Q}{R}} \right)$
  • B$\frac{1}{{4\pi { \in _0}}}\left( {\frac{Q}{R} + \frac{q}{r}} \right)$
  • C$\frac{1}{{4\pi { \in _0}}}\left( {\frac{q}{r} - \frac{q}{R}} \right)$
  • D$\frac{1}{{4\pi { \in _0}}}\left( {\frac{q}{R} - \frac{Q}{r}} \right)$
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