A conducting sphere $A$ of radius $a$, with charge $Q$, is placed concentrically inside a conducting shell $B$ of radius $b$. $B$ is earthed. $C$ is the common centre of the $A$ and $B$. 

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  • A

    The field is a distance $r$ from $C$, where $a \leq  r \leq  b,$ is $\frac{1}{{4\pi \,{\varepsilon _0}}}\,\frac{Q}{{{r^2}}}$

  • B

    The potential at a distance $r$ from $C$, where $a \leq  r \leq  b,$ $\frac{1}{{4\pi \,{\varepsilon _0}}}\,Q\,\left( {\frac{1}{r}\, - \,\frac{1}{b}} \right)$

  • C

    The potential difference between $A$ and $B$ is $\frac{1}{{4\pi \,{\varepsilon _0}}}\,Q\,\left( {\frac{1}{a}\, - \,\frac{1}{b}} \right)$

  • D

    all of the above

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