b
\({C_{cm}}\,\, = \,\,\,\,\frac{{{Q_{net}}}}{{{C_1}\,\, + \,\,{C_2}}}\, = \,\,\frac{{{Q_{net}}}}{{4\pi \,{ \in _0}\,\,{R_1}\,\, + \;\,4\pi { \in _0}\,\,{R_2}}} = \,\frac{{{Q_{net}}}}{{4\pi \,{ \in _0}\,\left( {{R_1}\,\, + \,\,{R_2}} \right)}}\,\)
\(\, = \,\,\frac{{150\,\, \times \,\,{{10}^{ - 6}}\,\, \times \,\,9\,\, \times \,\,{{10}^9}}}{{\left( {10\,\, + \;\,20} \right)\,\, \times \,\,{{10}^{ - 2}}}}\,\, = \,\,4.5\,\, \times \,\,{10^6}\,\,volt\)