Shown below are two concentric spherical shells of negligible thicknesses and radii $R_{1}$ and $R_{2} .$ The inner and outer shell carry net charges $q_{1}$ and $q_{2},$ respectively, where both $q_{1}$ and $q_{2}$ are positive. What is the electric potential in the regions (a) $r < R_{1},$ (b) $R_{1} < r < R_{2}$
and (c) $r > R_{2} ?$