An empty thick conducting shell of inner radius $a$ and outer radius $b$ is shown in the figure. If it is observed that the inner face of the shell carries a uniform charge density $-\sigma$ and the outer surface carries a uniform charge density $\sigma'$,and a point charge $q_A$ is placed at the center of the shell,then choose the correct statement$(s)$.

• A
  The charge $q_A$ must be positive.
• B
  The charge $q_A$ must be negative.
• C
  The magnitude of charge $q_A$ must be $4\pi \sigma a^2$.
• D
  $A$ and $C$ both.