If $\hat{a}$ is a unit vector such that $(\bar{x}-\hat{a}) \cdot (\bar{x}+\hat{a}) = 8$,then $|\bar{x}| = $

  • A
    $\pm 3$
  • B
    $2 \sqrt{2}$
  • C
    $3$
  • D
    $\pm \sqrt{7}$

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