If a line makes angles $\alpha, \beta, \gamma$ with the positive direction of $x, y$ and $z$-axis respectively,then $\cos^2 \alpha + \cos^2 \beta + \cos^2 \gamma$ is equal to:

  • A
    $1/2$
  • B
    $-1/2$
  • C
    $-1$
  • D
    $1$

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