$A$ 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