Three long concentric conducting cylindrical shells have radii $R, 2R$ and $2\sqrt 2 $ $ R$ . Inner and outer shells are connected to each other. The capacitance across middle and inner shells per unit length is: 
  • A$\frac{{\frac{1}{3}{ \in _0}}}{{\ln \,2}}$
  • B$\frac{{6\pi { \in _0}}}{{\ln \,2}}$
  • C$\frac{{\pi { \in _0}}}{{{\rm{2}}\ln \,2}}$
  • D
    None 
Diffcult
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