A neutral conducting solid sphere of radius $\mathrm{R}$ has two spherical cavities of radius a and $b$ as shown in the figure. Centre to centre distance between two cavities is $c . q_a$ and $q_b$ charges are placed at the centres of cavities respectively. The force between $q_a$ and $q_b$ is

• Correct answer
  A

  $\frac{1}{4 \pi \varepsilon_0} \frac{\mathrm{q}_{\mathrm{a}} \mathrm{q}_{\mathrm{b}}}{\mathrm{c}^2}$
• 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

Hence, the correct answer is option: 'A'.