Two thin concentric hollow conducting spheres of radii $R_1$ and $R_2$ bear charges $Q_1$ and $Q_2$ respectively. If $R_1 < R_2$,then the potential of a point at a distance $r$ from the centre $(R_1 < r < R_2)$ is

• A
  $\frac{1}{4\pi \varepsilon_0} \left( \frac{Q_1 + Q_2}{r} \right)$
• B
  $\frac{1}{4\pi \varepsilon_0} \left( \frac{Q_1}{r} + \frac{Q_2}{R_2} \right)$
• C
  $\frac{1}{4\pi \varepsilon_0} \left( \frac{Q_1}{R_1} + \frac{Q_2}{R_2} \right)$
• D
  $\frac{1}{4\pi \varepsilon_0} \left( \frac{Q_1}{R_1} + \frac{Q_2}{r} \right)$