An object is kept at a distance of 12 cm from a thin lens and image formed is real. If the object is kept at a distance of $8\;cm$ from the same lens image formed is virtual. If the size of image formed is equal length of lens will be :

Question

$(a)\;15\;cm \\ (b)\;17\;cm \\ (c)\;10\;cm \\ (d)\;8\;cm$