Coordinate planes and the planes $\pi_1, \pi_2, \pi_3$ which are respectively parallel to $YZ, ZX, XY$ planes at distances $a, b, c$,form a rectangular parallelepiped. $d_1$ is a diagonal of the face on the $XY$-plane not passing through the origin,and $d_2$ is a diagonal of plane $\pi_2$ coterminous with $d_1$. If none of the coordinates of the vertices of the parallelepiped are negative and the angle between $d_1$ and $d_2$ is $\theta$,then $\cos \theta=$

  • A
    $\frac{a^2}{\sqrt{a^2+b^2} \sqrt{a^2+c^2}}$
  • B
    $\frac{a}{a^2+b^2+c^2}$
  • C
    $\frac{\pi}{2}$
  • D
    $\frac{a^2}{\sqrt{a^2+b^2} \sqrt{b^2+c^2}}$

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