$A$ 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 $B$.

• A
  The electric field at 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 $a \leq r \leq b$,is $\frac{1}{4\pi \varepsilon_0} Q \left( \frac{1}{r} - \frac{1}{b} \right)$.
• 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.