$A$ neutral conducting solid sphere of radius $R$ has two spherical cavities of radius $a$ and $b$ as shown in the figure. The centre-to-centre distance between the two cavities is $c$. Charges $q_a$ and $q_b$ are placed at the centres of the cavities, respectively. The force between $q_a$ and $q_b$ is:

  • A
    $\frac{1}{4 \pi \varepsilon_0} \frac{q_{a} q_{b}}{c^2}$
  • B
    $\frac{1}{4 \pi \varepsilon_0} q_a q_b\left(\frac{1}{a^2}+\frac{1}{b^2}\right)$
  • C
    zero
  • D
    insufficient data

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