The diagonals of the parallelogram whose sides are $lx + my + n = 0$,$lx + my + n' = 0$,$mx + ly + n = 0$,and $mx + ly + n' = 0$ include an angle of:

  • A
    $\frac{\pi}{3}$
  • B
    $\frac{\pi}{2}$
  • C
    $\tan^{-1}\left(\frac{l^2 - m^2}{l^2 + m^2}\right)$
  • D
    $\tan^{-1}\left(\frac{2lm}{l^2 + m^2}\right)$

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