If $x,\;y,\;z$ are real and distinct, then $u = {x^2} + 4{y^2} + 9{z^2} - 6yz - 3zx - zxy$ is always

  • [IIT 1979]
  • A

    Non-negative

  • B

    Non-positive

  • C

    Zero

  • D

    None of these

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