Two spherical conductors A and B of radius a and b (b > a) are placed in air concentrically B is given charge + Q coulomb and A is grounded. The equivalent capacitance of these is [MP PMT 2001]

• A.            \[4\pi {{\varepsilon }_{0}}\frac{ab}{b-a}\]       
• B.            \[4\pi {{\varepsilon }_{0}}\,(a+b)\]
• C.            \[4\pi {{\varepsilon }_{0}}b\]
• D.            \[4\pi {{\varepsilon }_{0}}\frac{{{b}^{2}}}{b-a}\]