A system of lens consists of two convex lenses of focal length $f_1= 30\;cm$ and $f_2 =20\;cm$. Separated by a distance 'd'. When a parallel beam of light is incident on the first lens after two refraction the resultant rays come out parallel then the distance 'd' is

Question

$(a)\;40\;cm \\ (b)\;50\;cm \\ (c)\;30\;cm \\ (d)\;60\;cm$