$A$ and $C$ are concentric conducting spherical shells of radii $a$ and $c$ respectively. $A$ is surrounded by a concentric dielectric layer of inner radius $a$,outer radius $b$,and dielectric constant $k$. If sphere $A$ is given a charge $Q$,the potential at the outer surface of the dielectric (at radius $b$) is:

• A
  $\frac{Q}{{4\pi {\varepsilon _0}kb}}$
• B
  $\frac{Q}{{4\pi {\varepsilon _0}}}\left( {\frac{1}{a} + \frac{1}{{k\left( {b - a} \right)}}} \right)$
• C
  $\frac{Q}{{4\pi {\varepsilon _0}b}}$
• D
  None of these