Two concentric sphere kept in air have radii R & r . they have similar change and equal charge density σ. The electric potential at their common centre is? ( MHT - CET ~ 2014 )

  \[ a ) \frac { \sigma (R + r) } { \epsilon _{ o } } \]    \[ b ) \frac { \sigma (R - r) } { \epsilon _{ o } } \]    \[ c ) \frac { \si...