Two concentric shells have masses $M$ and $m$ and their radii are $R$ and $r$ respectively,where $R > r$. If $x$ is the distance from the common centre,what is the gravitational potential at a point for which $r < x < R$?

• A
  $ - G\left[ \frac{M}{x} + \frac{m}{r} \right] $
• B
  $ - G\left[ \frac{M}{x} - \frac{m}{r} \right] $
• C
  $ - G\left[ \frac{M}{R} + \frac{m}{x} \right] $
• D
  $ - G\left[ \frac{M}{R} - \frac{m}{x} \right] $