There are four concentric shells $A, B, C $ and $D $ of radii $ a, 2a, 3a$  and $4a$ respectively. Shells $B$ and $D$ are given charges $+q$ and $-q$ respectively. Shell $C$ is now earthed. The potential difference $V_A - V_C $ is : 

  • A

    $\frac{{Kq}}{{2a}}$

  • B

    $\frac{{Kq}}{{3a}}$

  • C

    $\frac{{Kq}}{{4a}}$

  • D

    $\frac{{Kq}}{{6a}}$

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