Let $\log _a b=4, \log _c d=2$, where $a, b, c, d$ are natural numbers. Given that $b-d=7$, the value of $c-a$ is

  • [KVPY 2009]
  • A

    $1$

  • B

    $-1$

  • C

    $2$

  • D

    $-2$

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