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
  • A$4\pi {\varepsilon _0}\frac{{ab}}{{b - a}}$
  • B$4\pi {\varepsilon _0}\,(a + b)$
  • C$4\pi {\varepsilon _0}b$
  • D$4\pi {\varepsilon _0}\frac{{{b^2}}}{{b - a}}$
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