The vector $\overrightarrow P = a\hat i + a\hat j + 3\hat k$ and $\overrightarrow Q = a\hat i - 2\hat j - \hat k$ are perpendicular to each other. The positive value of $a$ is

  • [AIIMS 2002]
  • A

    $3$

  • B

    $4$

  • C

    $9$

  • D

    $13$

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  • [JEE MAIN 2023]