Four charges equal to $-Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium, the value of $q$ is

  • A

    $-\frac{Q}{4} (1 + 2 \sqrt 2 )$

  • B

    $\frac{Q}{4} (1 + 2 \sqrt 2 )$

  • C

    $-\frac{Q}{2} (1 + 2 \sqrt 2 )$

  • D

    $\frac{Q}{2} (1 + 2 \sqrt 2 )$

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