$A$ parallel beam of light is incident on a system of two convex lenses of focal lengths $f_1 = 20\, cm$ and $f_2 = 10\, cm$. What should be the distance between the two lenses so that the rays after refraction from both the lenses emerge as a parallel beam?........$cm$

  • A
    $60$
  • B
    $30$
  • C
    $90$
  • D
    $40$

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