A convex lens is used in the experiment of conjugate focus method where image of the object is formed in the same place for two positions of the lens. The size of image in 1st position is $4\;cm$ and in the second position is $16 \;cm$ . The length of the object is

Question

$(a)\;6\;cm\\(b)\;8\;cm\\(c)\;12\;cm\\(d)\;16\;cm$