Coordinate planes and the planes \(\pi_1, \pi_2, \pi_3\) which are respectively parallel to \(\mathrm{YZ}, \mathrm{ZX}, \mathrm{XY}\) planes at distances \(\mathrm{a}, \mathrm{b}, \mathrm{c}\), form a rectangular parallelpiped, \(\mathrm{d}_1\) is a diagonal of the face on \(\mathrm{XY}\)-plane not passing through origin and \(\mathrm{d}_2\) is diagonal of plane \(\pi_2\) coterminous with \(\mathrm{d}_1\). If none of the coordinates of the vertices of the parallelpiped are negative and angle between \(\mathrm{d}_1\) and \(\mathrm{d}_2\) is \(\theta\), then \(\cos \theta=\)

1. A \(\frac{a^2}{\sqrt{a^2+b^2} \sqrt{a^2+c^2}}\)
2. B \(\frac{a}{a^2+b^2+c^2}\)
3. C \(\frac{\pi}{2}\)
4. D \(\frac{\mathrm{a}^2}{\sqrt{\mathrm{a}^2+\mathrm{b}^2} \sqrt{\mathrm{b}^2+\mathrm{c}^2}}\)